a couple of PD games come to mind.
I had a lot of fun with:
1) Drip - a kind of pacman style maze game that was hilarious to even watch someone play. As I recall it didn't like accelerated Amigas, and use of a downgrader was essential for play on my 030 accelerated Amiga 1200.
2) Megaball - a breakout clone by Ed Mackey, initially shareware and then placed into the public domain. Lots of fun!
