I also second the recommendation to look at Freewarepalm.com. Great stuff can be found there.
The free games that I have on my Tungsten T3 are:
Patience 3.0.1 - Simply one of the best solitare collections ever for Palm OS systems.
Mulg II q - Move your marble around different mazes. Each maze is a puzzle to solve.
Pilot Mines - Implementation of Minesweeper for the Palm.
Cave Run - Avoide the rocks. Really addictive.
Yahdice - A really good yahtzee clone.
Games I’ve paid for:
Bejeweled and Bejeweled 2 - Can be downloaded from http://www.astraware.com, both are excellent games and incredibly addictive.
Euchre - From http://www.seahorsesoft.com - A great euchre game for the palm.
I hope this gives you a place to start looking around.