Mac OS X Server 1.0, released on March 16, 1999, is the first operating system released into the retail market by Apple Computer based on their acquisition of NeXT. It followed the Rhapsody series of developer releases of what was to be known as Mac OS X. Although OS X Server 1.0 had a variation of the Platinum "look and feel" from Mac OS 8, it is based on the OPENSTEP (and thus, NeXTSTEP) operating system instead of the classic Mac OS, giving users a preview of the future operating system Mac OS X. It was replaced by a version based on Mac OS X v10.0 in 2001.(Source: Wikipedia)
Officially it runs on some PowerPC Macs (list of supported computers is here: http://support.apple.com/kb/TA28250).