I blogged before about XCOM: Enemy Unknown, a game released in 1994 which is still today heralded as the first (and so far only) example of a real time strategy (RTS) game which combines turn based strategy seamlessly to make a very accessible, enjoyable game. Easy to play but at the same time, involves a great deal of thought to get right.
I will start off with the direct clones, games that, either as a result of an open source community or a big software house have tried to recreate the magic.
UFO 2000
Perhaps the original, the one that started it all. The majority of the work was done by a Russian programmer who did a magnificent job of laying down the basic framework. With the 0.2.x builds, you could randomly generate a terrain and populate it with soldiers playing a very basic hot-seat style multiplayer game. Since then, a great deal of work has gone into this project to improve and expand on this framework. It has become a fairly stable multiplayer game with its own hosted server which is normally always populated and even has its own (rough) league table based on the completed games.
The only problem I have is that, in an attempt to shed all non F/OSS content, they have dropped the requirement for the UFO: Enemy Unknown graphics. What this means is that you can play with an entirely community made set of artwork / weapons. This is great in some ways and a lot of people have contributed both terrain, weapon sets and armour artwork but this is the project’s greatest problem at present. There are so many different weapon sets you can choose from (as well as armour and graphics) that the game feels as though it has abandoned its routes in the xcom universe. Personally I feel that the game should stick to what it is good at (being a multiplayer UFO:EU clone) and have these community driven skins/models on a completely different server.

This is definitely the most mature project (it supports both UFO: Enemy Unknown AND Terror from the deep graphic sets) infact, most telling is what is currently NOT supported:
Having recently added flying units and deloyable aliens such as the Floater, Ethereal and Snakeman making this clone the most feature complete around at present and definitely one I would strongly recommend.
UFO: AI (Alien Invasion)
Is another fairly recent project that has come forwards in leaps and bounds in the last few years. Based on the open source Quake 2 (known internally as the id Tech 2) engine, the team behind UFO:AI have spent considerable time making the maps modular and converting this first person engine into a RTS style one. While at the low level, the engine is still a FPS, the camera has been dramatically modified as to is the engine’s ability to handle dynamic map production through the stitching of smaller map units. I have played a few of the builds the team has released in the last few years and overall I am impressed.

Unfortunately the game suffers from a very jagged feel, unit movement and shooting phases do not feel fluidic and it is quite hard to beat the aliens who seem to have an unfair advantage coded in. Given the limitations of the engine they started with, they have made huge progress but I am unconvinced this project will mature much further.
X-Force: Fight for Destiny
This is one of the clones of Ufo: Enemy unknown that I really enjoyed playing, although very rough around the edges you can see the amount of planning that has gone into this game and it is fun to play. The moot point is that the native language of the development team is German, this would not be an issue if it was not for the very poor job someone did translating some of it (the rest of the clone is still in German.) There are several key things which make this game quite enjoyable, firstly the game is uniquely different, but easy for a XCOM veteran to pick up and play with minial adjustment, and secondly, the way in which destructible terrain has been implemented. In a way it feels a lot more like Apocalipse but in the UFO: EU universe which makes it quite entertaining. I have not tried the more recent builds but this is one I would definitely recommend you try.
Project Xenocide
This clone has wowed with spectacular renders and very grand plans, unfortunately this project started life as a kind of Geoscape (the real time strategy part of UFO: Enemy Unknown) tech demo and a few years on that shows. Whilst the Geoscape looks quite mature, the extent to which the tactical aspect has been developed can been seen in their 2007 tech demo video. This is not a serious clone at the moment because it is missing the most vital part of UFO:EU. It would be interesting if the team from Project Xenocide merged with the UFO 2000 team, although that is unlikely as it is clear they have their own plans for the Tactical element of the game. Sadly at the moment all they are is a fairly dull tech demo.
UFO Pocket PC
This is more of a port than a clone although technically the latter is the case. The Russian team (seeing a trend here?
) have out of nowhere completed a very comprehensive clone for the Windows Mobile Pocket PC platform. I barely use my PDA but I tried it out and was pleasantly impressed. In my mobile was Pocket PC (its Symbian) then I would carry thisw around with me everywhere, the port is that good. The only minor niggle is the look system for the soldiers. Due to the touch screen only having a left click implementation, turning your soldiers is a bit cumbersome.

Whilst a lot of the GUI/ management screens are unique to this port, the game itself has been remade with astonishing accuracy. This is definately one to try if you have a Pocket PC.
UFO: Extraterrestrials
Definately one of the most polished clones, this game is a commercial offering set in a similar universe (although not the same to avoid royalty costs to the present holder of the XCOM trademark) to the originals. The Geoscape and Tactical elements are almost identical but with a variety of subtle differences. I got bored about half way through this game as it seemed to lack any real innovation. Whilst graphically it is very good, the combat seems a little meek compared to the vicious bark of the original Heavy Plasma rifle. That being said, it has a variety of game modes and the aliens are not stupid requiring a cautious commander versed in tactical thinking.

I really like the way combat is depicted on the Geoscape, whilst in the original time ’stopped’ during an interception whilst your interceptor battled the UFO in the popup screen, the battle takes place in all its glory right on your Geoscape which I was very impressed by. There is no doubt about it, this is a game to buy as it will provide hours of enjoyment.
The Altar Games : Aftermath, Aftershock, Afterlight
Of course, no list of XCOM universe remakes would be complete without a nod to Altar Entertainment and their trilogy of games. Aftermath came first and, whilst it was a clone of UFO:EU it has a completely different storyline which is actually quite good.
The game starts in a movie theater with a strange dust floating in the projected light, the camera tilts and you everyone is dead, suffocated by strange, alien spores. The game starts by telling you ‘yesterday’ and alien craft arrived in orbit, all communication failed and it began unleashing spores into the atmosphere. There spores blanketed the world and suffocated most of the human race. Only a select few remain. (Enter player)
The Geoscape is very different to classic UFO:EU/Xcom games in that, you start off with a small sector of the globe under your control. This is your base, you must expand your territory as you go along by completing missions and ridding nearby sectors of aliens, both mutants and the intelligent ‘grey’ like species which look suspiciously like stectoids. As you gain territory, you come across other humans who can join your ranks as soldiers. You also perform UFO intercepts and raiding missions (although you do not learn how to enter the UFOs till later.) Overall the game is extremely enjoyable and the best of the three games from Altar, however it is crippled later on by the aliens developing a second stage to their technology which is so powerful, even an experienced play could have difficulty getting more than a 1:1 kill ratio (aliens for your soldiers) which any die hard fan will agree is too hard, especially given the scarcity of volunteers.
I must mention the Geoscape interception of UFOs which is done in a unique and fun way. The battle is not controlled by the player but in the form of a brief (20 second) video of the dog fight your interceptors have with the UFO(s). This is both fun and exciting as there are a number of different outcomes which all have their own video sequence.
I only briefly played the second two games from Altar as I found them very boring and without any real redeeming features after a few hours. I do recommend Aftermath but not the other two.
I want to finish by mentioning the games that are set in the XCOM universe, but are either a completely different genre.
Clancy: The XCOM Story
This is a WIP first person shooter mod for Half Life 2 set in the XCOM universe. It appears to be a linear story driven campaign in the fairly early stages although the project has a great website and introductory video which I highly recommend. Ever since seeing the damage the UFO did in Half Life 2: Gary’s Mod I have been longing for a XCOM mod. This *could* be the answer, we shall have to see.
If nothing else, I highly encourage you to watch their preview movie on their website.
XCOM: A Last Hope
This is a multiplayer first person shooter, also utlising the Half Life 2 engine. It looks as though the mod creators are aiming at an XCOM themed Counter Strike style game and although very much a WIP, they have released several builds. This is not one that I have tried, but it does look interesting if in need of graphical polish.
Although in truth, I should really add Terror from the Deep and Xcom Apocalipse to this list, as well as the other games officially released by the XCOM franchise, but to be honest they do not really feature on my list. The reason for this is Terror from the deep, whilst it has a lot of interesting tweaks / additions is just UFO:EU with a fresh coat of paint for me. Xcom Apocalipse is a great game, but I treat it as a new game (like the Altar series.)
So that is it, for the moment! If I have missed any games out feel free to comment and berate me
UPDATE : 05/06/08 : Corrected a few spelling mistakes. Also, UFO Pocket PC appears to have gone offline. The domain at which it was hosted is not longer available which is a shame. It was definitely something worth trying out if you owned a pocket PC.
UPDATE : 24/06/08 : URL problem seems to have corrected istelf. Pocket PC UFO is back.