A collection of freeware games for the Mac:

http://shinh.skr.jp/osxbin/
Titanion, Gunroar, Torus Trooper, A7XPG, Tumiki Fighters, PARSEC47
isshiki's games: Area 2048

Shmups
Noiz2sa and rRootage
Grid Wars
Gamma Bros
Alien Abduction (Defender clone)
Space Giko Harriya-n (Space Harrier clone)
ToJam Thing

Arcade
Millenipede (Centipede remake)
flOw
XIQ
Frets on Fire
Swarm Racer
Stair Dismount
Pogo Sticker
Tremulous
Route 960
Exolon DX
Super Obliteration
Heads Over Heels

Strategy
The Ur-Quan Masters (Star Control 2 remake)
FreeCol (Colonization remake)
Battle for Wesnoth
UFO: Alien Invasion
Gate 88

Platformers
N
Cave Story (translation patch)
Alex the Allegator 4
The Goonies 20th Anniversary

Adventure
Facade
Narcissu
Nearly Departed

Puzzlers
Rooms
Enigma (Oxyd remake)
Rocks'n'Diamonds (Boulder Dash, Emerald Mine, Supaplex and Sokoban remake)

Apple's Mac OS X games page has about 600-odd titles to download.