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 This has been superceded by _(Mac OS X).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.