Multris is a Tetris clone for Mac OS 8 and 9, released as shareware in 1999. Last I checked, it ran fine under Mac OS X Classic, but you need to find some old hardware (or an emulator) for it.
It features, in addition to a regular Tetris mode, hexagonal and triangular pieces, two-player mode, a computer AI and a piece editor to create your own sets of tetris pieces.
Multris is no longer sold as shareware, you can register it with the name "Multris" and code "50049246".
You can download Multris 1.0.1 as a StuffIt archive (100 kB).