Multiplayer games I can recommend:
Mario Kart Wii (fun for up to 4 players if your TV is big enough), Boom Blox (especially the jenga and shuffleball variants), Mario Power Tennis (it's a GC remake, but it's cheap and the new controls rock)
Games that I recommend, if you don't mind some minor flaws
Guitar Hero World Tour (lyrics are a bit hard to read, if a single player screws up the song ends for the whole band), Worms: A Space Oddity (the number of weapons has decreased quite a bit since the PC versions), Mortal Kombat Armageddon (good game, but some of the movements for the specials moves are a little too similiar and take some practice to pull off)
Games I can not recommend at all:
Deca Sports (boring as abyss)