One interesting and unheralded feature of the Wii is the fact that it plays Nintendo GameCube titles. (It also plays older games that you can buy online from the Nintendo 64, NES and other Nintendo systems through the "virtual console" feature, but I haven’t played around with those yet.)
We are getting a ton of mileage out of the old GameCube titles, many of which have been around a while so you can pick them up used or at a discount. Mario Kart Double Dash is totally ridiculous. Lego Star Wars has eaten up a ton of our time in the past few weeks, and we picked it up for just $19.95. The part at the end of Episode III where Queen Amidala gives birth had us in stitches but I won’t give the gag away. Every minute I play Lego Star Wars I think to myself "this is more fun than the actual movies were".
Both games are a pretty good entrée into the world of console gaming for little ones (my main motivation for getting the Wii was to have something to play with my five-year-old girl).