Glest is a 3D real time strategy game with beautiful graphics and sounds. Please see glest.org for details. It's a little like Warcraft III, real-time strategy (resources, attack, etc). It's free and can be played on a network.
Maybe if more Mac people use it, they will contribute some cool modifications to the graphics and sounds to help it evolve. The source code is available at the developer's site.
Universal Binary (PPC/Intel)
• CPU: Intel or PowerPC (G4 or above)
• OS: Mac OS X 10.4 (Tiger) or later
• Disk space: about 110MB