There's a great new game out called External Hard Drive. Longest play time of any game I know of!
Anyway...
Mario Kart lasts you a really long time as long as you have Wi-Fi. Getting all the unlockable characters and vehicles takes a lot of skill and time, but the Wi-Fi can last you forever. I played that game hardcore for about 3 weeks straight, and I'm someone who loses interest very easily.
The Conduit is quite good. The Single Player is quite average but it's got some great online play. It's definitely no Xbox Live killer but it's still really damn close.
If you're not too fond of The Conduit but want a FPS game, there's CoD4 and CoD5 (I personally prefer CoD4) and Medal of Honor: Heroes 2 (which is only like $30).
NMH isn't long by any means, but it's such an amazing game and it's only $20.
You can always get Tetris Party on WiiWare. $10 and you'll get an amazing Tetris game. It's got so many modes, not to mention a fantastic Vs. Mode for both online and offline play.
I heard Animal Crossing can last you for a really long time but I really don't know. I just found it too slow for me.
If you liked Metroid Prime I'd get the Metroid Prime Trilogy. Each game is quite long on it's own (like 20+ hours, I think), and you've got all three of them here (as well as the MP2 multiplayer, which I personally liked).