DoveCot Mail (10.6), LDAP (User Management), iChat server (jabberd), VPN, Podcast, Wiki, HTTP... etc.
You can also set up Netboot, so that you can network boot a Mac and restore an image to your machine over the network. (We use this a TON!).. Try 50+ MacBooks at a time (separate network on separate switches).
http://www.apple.com/server/macosx/features/
You get a lot of functionality OOTB with a single 1U box. Now that they've taken the XServe away, I don't know what to think about Apple's Enterprise solutions.. Mini isn't powerful enough and the Mac Pro can't be rack mounted. Unless they're going to return into Enterprise with some cloud hosted system (NC Datacenter) but for our configuration it wouldn't make sense to do this.. (6,000+ Users).
I guess you could one-off some Mini for each application and have one Mini act as the Authentication server... wasteful though..