http://www.die2nite.com/
A recent review over at RPS piqued my interest in this title. Given that the title is by invite only (closed beta), hunted around the net for a key and ended up emailing the developers directly... who seemed more than happy to hand one over.
It's quite an interesting take on the "you have X actions per day" theme, dropping 40 survivors into a cooperative game, tasked with building up the town's defenses, scavenging for building materials and generally trying to survive. Every night (the game runs in real-time) at midnight, the zombie Horde attacks. Anyone outside the walls dies. If the gate isn't closed, the town gets overrun. If the town defenses aren't up to the task, some inhabitants don't survive the night. Next morning, dump those corpses outside the gates or tomorrow night you'll have zombie on the inside.
There is a fair bit of depth involved, much more so than in most other browser games I've played - do I use that wooden plank to build up my own refuge, or donate it to the common cause to increase everyone's chances of survival? Do we build up the town wall to increase our defenses or build a new pump to increase water rations? Can I spare that food to allow my stupid neighbour to make it back to town? Each decision can be discussed in the town forums and if some people don't play well, they can be banished. If that doesn't work - hang them!
There's a bit of a learning curve, but there's always another town to try over again.
Might be worth seeing if Node could hit them up for a batch of keys?
