Snake Jump
If you want to lose several hours of your day without even realizing it, this is the game for you. Simple controls - just the left and right arrows. The usual mission, don't fall to your doom. The highest scores are all hacked, but if you get in the zone you can do pretty well.
Zombie 4
This Java point/click/WASD shooter is fantastic fun, though it's pretty easy to complete. The green critters are zombies, and they try to infect the entire population. You've got to be pretty quick to catch them or else you'll end up spending a good chunk of time killing everything that moves.
Neon
Cool retro space shooter with mouse-only controls - probably lots of fun on the Wii if it will run.