Ok, here's some games i quite like
SuperTux
go back to the classic 2D scrolling platform adventures with this addictive game.
Freeciv
A port of the classic Civilization game.
Battle For Wesnoth
An epic RPG, very extensive with add-on scenarios and ability to create your own.
Head Over Heels
A classic from way back, now reborn on the Mac.
Blackjack International
Great casino card game - pit your wits against the the dealer and four other AI players.
Otis
Addictive puzzle game.
Quinn
Tetris on your Mac.
Dinky Dungeon
Adventure RPG game, nintendo style.
Transport Tycoon
The classic game brought to osx.
Open Arena
Game based on Quake 3 engine.
JSokoban
The classic Mac puzzle game.
Quackle
Play scrabble against AI opponent.
GeoParty
Trivia game in the style of Double Jeopardy
Spell-O-Gram
something for the kids - help them with their spelling.
JewelToy
An interesting and enjoyable puzzle game.
PingPop
Interesting, and addictive take on the breakout/Arkanoid game.
Pac The Man X
Pacman for your mac.
Flashback
The old, and excellent, Amiga game emulated on your mac. Check out
this site for more screenshots and dl too.
Enigma
Puzzle game.
Big Box of Blox
An interesting game, not got into this yet but i'll keep plugging away - a new take on the tetris style games.
Obviously there are a load more I could have listed, these are just the ones I have been interested in enough to download and play.
Some nice - "free to go so far" then option to pay games
Midnight Mansion
Awesome return to classic retro 2D side scrolling platformers.
Titan Attacks
Space Invaders rule!
Argonaut
Nice take on the Asteroids game.
Rocochet
Really nice Breakout/Arkanoid style game with a twist.