The server version of Tiger will offer weblog hosting with Blojsom. As far as I know Apple servers are used mostly in office environments. With Safari RSS and Blojsom Apple is becoming a very interesting k-logging environment. Paolo Valdemarin: Tiger k-logging
And an iChat server based on Jabber (well, it's a Jabber server, so it works with all Jabber-compatible clients as well). And of course, both the IM server and the blog server both tie into Open Directory for user accounts and authentication. Like Scott, I want iChat to start supporting arbitrary SIP servers as well. Somehow, the fact that there has been absolutely no word out of Apple about support for SIP makes me think they're up to something.
Check the server preview for all the other Tiger Server stuff.