Animal Crossing: Wild World - It never ends.
Baten Kaitos - I'm about an hour and a half in.
Pikmin 2 - Found it for $25 in March, and I like it more than I thought I would. I'm 2:22 in.
Tales of Phantasia - I'm 15 hours in.
Tetris DS - I play this at random intervals.
Yoshi's Island (Super Mario Advance 3, Player's Choice) - The music got nerfed, but the gameplay still owns. And I mostly got this for the extra levels anyway, so I'm happy.
So, yeah. I should seriously start beating games completely before starting new ones, as BK, Pikmin, ToP, and Yoshi's Island are a balancing act. I'm not even going to touch Mario and Luigi: SS for now...