There's a great preview of the features in the upcoming Leopard edition of Mac OS X Server here at Computerworld (First look: A guided tour of Apple's Leopard Server OS).

OS X Server has been my preferred server platform for the last few years. It provides all of the functionality I need: unified directory, DNS, DHCP, email service, database server, file serving (over NFS, AFP and SMB), web server and a few others I'm sure I've forgotten. There's also a very cool administration toolset.