If you played much of the fine Warcraft III mod Night of the Dead, then The Red Solstice [official site] should look mighty familiar. That's because it heavily draws inspiration from Night of the Dead, and one of Red Solstice's leads worked on a NotD variant. If not: 1) you missed out; 2) I'll explain.
The Red Solstice is a top-down tactical shooty action-RPG about scavenging supplies, completing missions, and gunning down alien hordes to survive hour-long missions in singleplayer and co-op modes. After one year on Steam Early Access, it has now properly launched.
Something bad has happened on Mars. Really, really bad. Sent in to find out what, you and your space marine chums find hordes of terrible monsters. Each round lasts one hour of real time, during which you'll roam around big maps collecting gear to help with the fight, completing randomised objectives, levelling up and unlocking new abilities (it has 8 classes), and shooting so many monsters. I haven't played Red Solstice yet, but I really enjoyed playing Night of the Dead in WC3 so I'll see if I can round people up again for this.