For games you can't go wrong with Bejeweled from Astraware. You can try out the original online version (called Diamond Mine) for free at
http://www.popcap.com.
If you grew up playing Lucasarts adventure games and still have the disks/cds for them you have to give pocketScummvm a try. It'll take the data files from your games and run them on your ppc. Bonus: You can download Beneath a Steel Sky (cd or flippy disk version) for free at the
http://scummvm.org website. Just make sure you read all the documentation.
PocketNester (
http://jetech.org ) allows you to play NES games provided you have the roms.