4 player co-op shooter

Alien Swarm                                                                                          $: Free to play (Steam)

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 Alien Swarm was developed by the same team behind a similarly titled Unreal Tournament 2004 mod.  The game is a top-down shoot em’ up with the camera angle set at 60 degrees.  Alien Swarm is a purely cooperative game where a team of four players must clear stages by completing various objectives.  Players do gain levels, unlock achievements, new equipment, and other features as they progress.  Unlike most free to play games, Alien Swarm is freeware which means there’s no cash shop or item mall to worry about.  Alien Swarm stays true to its mod heritage and allows players free access to its source code.  This means creative fans will be able to create additional content to supplement the official maps.  There are currently four classes and eight playable characters to choose from at the start of each mission.

 This was actually one of the first games that I played online with Bad rouches, its pretty fun, Source engine FTW !

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Some useful links:
http://www.youtube.com/watch?v=804u05RHxNk - Omer is back !

Download:

http://store.steampowered.com/ - The best way is to register on steam and download the game from there.