Does anyone know if there's a site that lists which controllers are compatible with each Wii game?

Sometimes I'm reluctant to buy certain games if I can't find out whether they're playable with a Gamecube pad or classic controller.

I'm not sure about websites, but if you look on the back of a game's case it tells you. If you're talking about WiiWare and VC, it tells you on the download page.

I buy all my games online and don't have a game shop anywhere near me so I very rarely get to look at the cases unfortunately.

gamefaqs.com typically has the back of a box scanned in for at least 1 region. You could try there.

I think it is most probably is incomplete, but on wikipedia, under input methods, it says what controllers that can be used.

Good point!

I recommend mobygames.com.

Faster than gamefaqs, easier to search, better and larger scans such as these:
http://www.mobygames.com/game/wii/super-mario-galaxy/cover-art/gameCoverId,98316/
http://www.mobygames.com/game/wii/no-more-heroes/cover-art/gameCoverId,106778/
http://www.mobygames.com/game/wii/kami/cover-art/gameCoverId,110253/

Search for game (upper right), click on "x more" under the front cover, select back cover.

Best game museum site with easiest search on the net imo.

I forgot about mobygames. I don't use them because they watermark everything, including the boxarts