Unvanquished is an open-source FPS game featuring team-based strategy and RTS elements, pitting humans against aliens in a futuristic setting.
https://forums.unvanquished.net/
Hi, the team behind the “Ryzom” MMORPG game had publicly released their artworks under the CC-BY-SA license:
Some artworks for insectoids are inspiring:
The dretch’s sister?
The granger’s cousin?
A flying monster?
That can give ideas for redesigned aliens or new aliens. 
Interesting. I like the monoceratops granger.
Those are actually some pretty nice designs.
Actually they released 3dsmax models along with the sketches.
License: http://forums.ryzom.com/showthread.php?t=34929
Humans:
http://media.ryzom.com/?query=homin%203d&start=20
http://media.ryzom.com/?query=3d%20karavan&start=0
http://media.ryzom.com/?query=3d%20karavan&start=60
Monsters:
http://media.ryzom.com/?query=3d%20monster&start=140
http://media.ryzom.com/?query=3d%20mob&start=1000
Space Ships:
http://media.ryzom.com/?query=3d%20karavan&start=140
Plenty more, but they don't all have screenshots.
The models aren't bad. However, they wouldn't really fit into our art style, and there's other things to keep in mind, like texture resolution and polygon count. I'm sure community mappers could make use of them, though.