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Instead of making a multiple-year gravedig of this old thread, I thought I'd start a new one.
Alien Swarm: Reactive Drop came out recently (about a month ago). It is an update to the game by different developers that really Valve should have made years ago, and went through Steam Greenlight.
Posts: 3,637
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My understanding of the difference is that it allows up to 8 players instead of 4, has Steam Workshop support, a bunch of new campaigns (only one campaign was the main reason Alien Swarm became boring), and some other misc. improvements