The Red Solstice is a tactical cooperative survival game taking place on Mars in a very distant future. It's headed for Steam Early Access on July 10, and will support up to 8-players for co-op modes across randomly generated maps. I don't know how I feel about it just yet since I obviously haven't played it, but it looks like it could be the basis for some good times online.
Customizable equipment, eight different player classes, and what looks like an emphasis on tactical play slanting toward the horror genre make The Red Solstice look pretty delectable. It might very well be worth checking out.