Assault Android Cactus really shines in local coöp!
On Friday evening, we slapped two PS3 pads and two Xbox pads onto a Linux machine, connected a projector, and had a 5-hour marathon session, with maybe 20 people rotating in and out at various times.
There was lots of shouting at the screen and at each other ("Battery! Battery!" "Kill the toasters!"), as well as a fair amount of trashtalking over the statistics on round completion.
AAC was one of my first exposures to Steam Early Access, and it's a shining example of how the system was meant to work. When I first began playing, they had implemented one game mode, the four main characters, a handful of levels, and one boss. Since that time, WitchBeam has continually blasted out new characters, levels, bosses, and game modes, while continuing to revise, rebalance, and polish existing content.
At the time of this edit, AAC is still in early access (update 8.5), but I'm already extremely happy with my purchase, and I can't recommend this game enough. Additionally, future WitchBeam titles will be unquestioned day-0 purchases for me.