The New Super Mario Bros. Wii was announced at E3 earlier this month. In addition to it being the first wide-screen Mario Bros, complete with new powerups (like helicopters and penguins) they're finally introducing 4 player coop to the franchise!
That's made me really excited for this game. Hearing more news about people who were able to play the demo, it seems like there's even a 2 vs. 2 mode where you play the level while trying to disrupt or kill the other team. At the end of the level you're ranked (a la Mario Kart) based on coins, powerups, number of deaths, etc. It appears they've brought a little Smash Bros. element into humble ol' Mario Bros.
And I, for one, can't wait.