I’d like to third Boom Blox. I got it exclusively for parties. Not only is it very easy to play, it’s easy to handle even with one or two Wiimotes, and accommodates four players. We usually just hand them around to whoever’s standing nearby. It’s very accessible to people who don’t normally play video games. Even my mom played with us. It is turn-based, but, you need to pay attention to see what other players have done; it’s not just passive and score-based like bowling or golf. Each player’s actions impact the other. Think supercharged Jenga.
I’ve never played it but isn’t “Resident Evil: Umbrella Chronicles” two player co-op? If so, that may be one to check out.
Any [del]zombies crawled out of their graves[/del] new recommendations since the thread’s 2008 debut? Since the OP is pretty close to what I’d ask (simultaneous co-op multiplayer) I thought resurrecting it would help with duplicate suggestions. The only thing I’d add is that one of the players is about 4.5, so ‘E’ ratings and fairly easy to enjoy are important. Aside from Mario Kart and Brothers, I just ordered Kirby and Rampage. Any other suggestions?