Just play games from around the year when your toaster PC would be considered a reasonable PC.
But some of my favorites are:
Wolfenstein Enemy Territory
Rainbow six 3
UT2004/99
Quake 3
Morrowind
NFS:U2
Flatout 2
Postal 2
Battlefront 2
FEAR
Cossacks: Back to War (actually, there are a lot of good older strategy games, it feels like the genre hasn't evolved that much and performance doesn't matter too much)
Penguin Arena (Fuck yeah this game is fun and almost anything can run it)
Or find opensource games, nearly all of them run on total wrecks of PCs.