A whole page of the Wii menu so far.
All 3 Streets of Rage
Gunstar Heroes
Gain Ground
Link to the Past
Super Metroid
Mario Kart 64
Lylat Wars
Bomberman
Mean Bean Machine
Paper Mario
Most of the games I buy are for the multiplayer (although I still need to sort out having enough pads for four-player Mario Kart/Lylat Wars), apart from the two SNES games (and Paper Mario), which are just games I've never gotten round to playing fully.
This is as well as me resisting the urge to buy games I've already played to death but have trouble saying no to one more go on, like Ocarina, Mario 64, Mario World and the Sonics.
EDIT: Also, Bomberman is still the best multiplayer game on the Wii.