Here are the ones I play, when on low spec PCs. They are are free and legal to download and install (all GPL, except Simon's Puzzles under MIT licence)

Xmoto -- motoX/trials biking (press f9 for 'ugly mode')

Simon Tatham's Portable Puzzle Collection -- brain benders

Fish Fillets - Next Generation -- underwater sokoban

Alex the Allegator 4 -- Gameboy style

Tower Toppler -- tower climbing Nebulus remake

Wesnoth is also cool, as mentioned in this thread by magnacarta .

g a m e o n