This has seriously been my addiction for the past week or so.
So I heard about the game, downloaded the demo from the Xbox live arcade and immediately went to go buy some MS Points.
The game is called Castle Crashers, from the same folks who brought us Alien Hominid HD. Tom Fulp (webmaster of newgrounds) and Dan Paladin. You take a one of a variety of characters (which most of them you must unlock) and adventure through the world of castle crashers in a beat em up style adventure reminiscent of Golden Axe and other titles. There are also some RPG elements such as leveling up, stats points, magic and whatnot. And also a broad range of weapons and pets (which follow you around providing various bonuses) to collect.
The game is simply fantastic. And when I say simply, I actually mean it. The game is simple, meaningless time killing fun. You can also play with 3 other friends, which is a great experience, or take the adventure online with Xbox Live. There is also a player vs player mode.
Anyways, heres a video of the first few levels in 4 player coop mode: