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It's pretty much a rehash of XCOM (still keeps the RT world view, but turn-based tactical combat), but it allows multiplayer action (co-op, or head-to-head, with one player being the aliens, up to 6 players, 3 to a side). Based on a heavily modded Quake2 engine. Standalone, it runs on Windows, Mac, and some Linux distros.

It's free, having been released under the GNU GPL, CC or, for documentation, under GNU FDL. So it's a work in progress.

Just found it today, and I'm going to play it when I get home. Was a HUGE fan of XCOM, so I was pretty impressed when I saw the screenshots.