Mac OS version 8.5 is a streamline of Mac OS 8.1. It features more PowerPC optimization (60% faster). This was the first Mac OS to end integration of API and ROM. In the old days, ROM was cheaper than disk space, so Apple put bitchloads of API code in ROM. This continued for 15 years, for some reason, until Mac OS 8.5 came along. It also features Sherlock, this stupid Internet search tool.

You can upgrade from it to Mac OS 8.6 by a free download at Apple's FTP site.