Apple's old proprietary OS for its Macintosh 68k and PowerPC platforms. While not technically very innovative (not providing memory protection or robust multi-tasking), it provides the user with a very well-adjusted user interface experience. This has been superceded by Mac OS X.

If you look in other areas (not memory protection, etc.) MacOS is/was quite advanced in some of its capabilities. Just read through the Inside Macintosh Volumes.

This page is linked from: Preemption and Cooperation