A hearty second for Katamari (although it is hard for dexterity- and memory-challenged folks) and Lego Star Wars (the cinematic scenes especially are a riot, and the best part is there’s no dialogue whatsoever! XD).
Other recommendations:
Super Puzzle Fighter 2 Turbo - (two players) Crack puzzle game using super deformed Capcom sprites; the characters must use gems and orbs to build combos and use their special moves to send various colored blocks to the other player’s field. Features Dan Hibiki, my favorite Capcom character of all time. 
Taiko Drummer - (two players) Basically, DDR/Guitar Hero styled action, except for drumming fanatics. It is playable on a controller if you don’t want to spring for the drum, but controller-based play is harder. The only disappointment is that there aren’t more songs, and one of them features Britney Spears. :eek:
Dynasty Warriors - (two players) Any of them, really: the DW series, the Extreme Series, and the Empires series all feature the same historical period, approximately the same characters, and the learning curve isn’t too difficult. Although it does involve slicing up nameless soldiers (and the occasional low-level minion/famous general), the special moves are so outrageous that you tend to not take the game too seriously. (Where else would you be able to fight with fans, maracas, or giant popiscle sticks? And did I mention that there’s always at least one character who shoots LASERS? XD)
Or, if you’re a fan of Japanese history, you can try Samurai Warriors, which covers the Sengoku (warring states) period. I find that game a bit more disappointing, though, due to a combination of unfamiliarity with that particular era and the stupid maze stages that I always get horrendously lost in.
The Nippon Ichi RPGs–Pucelle Tactics, Disgaea: Hour of Darkness, Phantom Brave, and Makai Kingdom (one player)–all feature anime-styled super deformed sprites bashing the hell out of each other amidst pop culture references, jokes, and fan service up the wazoo. Word of warning: These games will eat up insane amounts of time. While most regular battles can be completed within minutes, there’s aspects of the game that cannot be accessed without plunking down a couple hundred hours’ worth of leveling up. (To give you an idea of how frighteningly addictive Disgaea is, the maximum level in the game is 9999…and then you can “reincarnate” the character back to level 1 with a bucketload of bonus stats and start the process over. :eeek: )
