The Wii is backward compatible with the GameCube.  With the Wii being a new system, there are not a lot of Wii games out yet.  Also GameCube games are cheap.  All of this adds up to the fact that Wii owners should look into buying some GameCube games.  With that in mind, Geek Idiot has published "The 40 Essential Gamecube Games".  Most GameCube games can be found for under $15.

We recently published an article on Donley Kong Jungle Beat which is an excellent purchase because it includes the Nintendo bongos.  We also recommend buying Animal Crossing.  This is one of the highest rated GameCube game.  Make sure you buy the "Players Choice" version as it contains a free GameCube memory card which can be used in the Wii.  You should be able to find it for under $20.

Update 01/03/2007: Cameron Banga has published a list of 20 must-have GameCube games , each item with a mini-review.