Castle Crashers is a high intensity, 4-player co-op, action hack and slash game. The game is all hand drawn and rendered in beautiful HD. The gameplay is very much like the old two and a half demensional side scrolling co-op fighting games like Teenage Mutant Ninja Turtles, X-Men, and The Simpsons arcade games.
I had a chance to play some 4-player Co-Op of Castle Crashers at the Penny Arcade Expo last year, and I gotta say - it was the game of the show. I liked the game so much I dropped a few ducets on the full four figure set.
There's also a new video from GDC08. While it doesn't show the co-op action, it worth a watch to see how the game will play. Now imagine that insanity with 4x the enemies and 3 more friends!