Quake

Quake is the mythical game created by John Carmack and the rest of the id Software folks, and solely responsible for an entire generation's vastly enhanced spatial visualization skills (as well as the uncanny ability to hit moving objects in mid-air with other moving objects).

Since the original Quake I source code is now freely available, there are a few interesting ports and extensions out there:

• Quake Revitalization Project - new textures and skins for GL clients.
• ezQuake - modern QuakeWorld client for all platforms.
• eQuake - all-in-one installers for Windows and Linux
• QuakeForge (Windows/Linux/UNIX, apparently dead)
• Fruitz of Dojo (Mac OS X, their TenebraeQuake client has recently become Universal).

• Nexuiz - a DM-oriented GPL conversion with 17 maps and a vastly improved graphics engine
• Tenebrae
• New Episode I Textures
• New Models

• qstat
• Only Mortal

• A Quake-style console for Mac OS X
• Wikipedia Article on Circle Strafing - utterly amazing by the very fact of its existence.
• IPv6 Quake, yes, an IPv6-enabled version. And no, it doesn't have longer rockets.
• The Quake Equation

If you absolutely must get at the original Quake files and have nothing but the original DOS CD (yes, it didn't run in Windows), DOSBox will let you "install" it in your Mac.