I was writing up a list of multiplayer indie games that look to be worth trying with a big group. This is the list I came up with:
Known Good:
- worth trying - http://www.crypticsea.com/anewzero/
 - 4 stars - http://www.playhawken.com/
 - 3 stars - http://sauerbraten.org/
 - 3 stars- http://www.steel-storm.com/ (not free)
 - 3 stars - frozen synapse (not free)
 - 4 stars- http://www.pixelprospector.com/openlierox/
 - 2 stars - http://www.warsow.net/
 - a billion stars - http://executedata.blogspot.com/search/label/tanktank
 
Looks Interesting:
- http://kag2d.com/en/
 - http://diaspora.hard-light.net/
 - http://www.pixelprospector.com/teeworlds/
 - http://www.pixelprospector.com/subvein/
 - shared computer only - http://www.pixelprospector.com/das-uberleben/
 - http://www.pixelprospector.com/wings-2/