store.steampowered.com/app/630/
Ignoring a certain dislike for the Steam platform that a certain person will certainly voice their opinion in this particular thread, (

) Valve have released this game, for free, to anyone that has a Steam account.
If you don't have a Steam account, that's free too - just install the client:
store.steampowered.com/about/
Anyway, the game:
This is a 4 player top-down objective based co-op bug hunt, where your team - a Soldier, Medic, Technician and Demolitions expert - must traverse alien infested installations and get out alive, achieving certain tasks as they proceed.
This used to be a popular UT2004 mod (
www.blackcatgames.com/asv1/index ) and all went quiet on development about 3 years ago. Quite clearly, this is when Valve hired the mod team and they have since developed this version.
Also released free, the game includes a Dedicated Server, and a full SDK - including mapping tools (one of which is a tile-based map generator) and the full source code.
If anyone wants me to join them, contact me on Steam - my steam username is "subenji".
Update
OK I've just played it a bit with my brother, and I have impressions!
1st - even though it was only the 2 of us, the game's difficulty scaled well - it didn't seem overwhelming, or that it was unbeatable without a full team. I will say though that a Technician is mandatory - and a Medic is a good idea too. For single player, the game has bots, but they won't fill up empty multiplayer slots.
2nd - it looks great! A lot of people think the Source engine is getting too old, but this game looks brilliant, and ran at full graphics settings smoothly on my 3 year old PC!
3rd - there's not a whole lot of content. There is 1 campaign, 5 maps long (there is a 'mod example' campaign too, but that amounts to just a basic tutorial along with the obvious example for modders) which will take about an hour or so with a full group to get through. They are certainly betting on a lot of modders making content for this, but personally, I don't have any doubts about that - there will be plenty for this.
4th - it's fun - very fun. I suggest everyone have a microphone, but you'll all have a blast!