If you're missing Apple's Xserve line of computers, then Sonnet's xMac Mini Server is one way to get your Mac back into the rack.
XServe Knowledge Base Articles
Apple Updates Xserve Software
Rebuilding XServe Arrays
The enterprise server will be discontinued January 31. As an alternative the Mac Pro will be offered with beefed-up specs and a copy of Mac OS X Server.
Apple upgrades Xserve, ships Xsan
XServe Reaction: Knowledge Base Articles
Troubleshooting Xserve: Server Monitor Workaround
Troubleshooting Xserve: Server Monitor Problems
New version of Parallels Server for Mac is aimed at companies standardizizing on Mac hardware and at cloud services providers eyeing Mac OS X capabilities.