If you like sports, Madden 07 on the DS is great.

You'll laugh at me, but for $20 "Cooking Mama" is a GREAT quick game. Zoo Keeper is just like Bejeweled but with stylized zoo animals and  a couple of new rules. If you played Animal Crossing on the GC, you already know what to expect on the DS.

Some of the old RPGs for the GBA are great, Golden Sun, Riva, Tactics Ogre, it all depends on what you like.

Big downside, the DS WiFi doesn't work with WPA, WEP only. So I have yet to go online with my DS. I'll probably spring for the DS Wifi adapter at some point, tired of entering in big hex strings into Windows WiFi settings and trying to connect with my DS. .