Hyperspeed is arguably the best Space Invaders-type shoot-em-up game for the Mac. It comes with 50 levels packed with fast, 60 fps action and stereo sound, and is fully compatible with MacOS 8 and 8.5. It requires a PowerMac to work.
You are the pilot of an Exodus SSC-12 spacecraft, with which you will fight off an alien invasion threatening to crush the Alliance of Unioned Planets. The SSC is at first equipped only with a plasma cannon, but soon you will get a number of great power-ups that will upgrade your cannons with tripple shots, grenades, and more.