IGN posted a list of the top 25 GameCube games. Which isn't only valuable for nostalgic value, given that the Wii has 100% compatibility with all GameCube games. So if you're looking for some good games to feed your Wii, this might be a good place to start.
Here's the list, which I've annotated with my own personal status (whether I own the game (*), rented/borrowed it (~), or never played it).
1) Metroid Prime *
2) Resident Evil 4
3) Super Smash Bros. Melee *
4) The Legend of Zelda: The Wind Waker *
5) Eternal Darkness: Sanity's Requiem *
6) Pikmin 2 *
7) F Zero GX ~
8) Prince of Persia: The Sands of Time *
9) Super Mario Sunshine *
10) Wave Race: Blue Storm
11) Animal Crossing *
12) Beyond Good & Evil *
13) Resident Evil ~
14) Soulcalibur 2
15) Metroid Prime 2 Echoes *
16) Paper Mario: The Thousand-Year Door *
17) Viewtiful Joe *
18) Star Wars Rogue Squadron II: Rogue Leader *
19) Timesplitters 2
20) Killer 7
21) Super Monkey Ball 2 ~
22) Battalion Wars
23) Baten Kaitos: Eternal Wings and the Lost Ocean
24) DK Jungle Beat
25) Metal Gear Solid: Twin Snakes
You can also get a similar list just by doing a review filter on GameSpot or GameSpy.
Some of the selections are a little odd, but in general it's a good list. And I'm happy to see that I have 13 of those 25 in my personal collection. Which if I run out of things to do with my Wii gives me a lot of great titles to fall back on.