MacOS X ships with what you'd expect most BSD UNIX systems to ship with, except for a command line filesystem dump/restore utility, and X11 .
All the usual daemons are there, and a bunch of other stuff can be easily compiled. Serveral X11 options exist - Tenon XTools if you want a commercial solution that runs rootless X right now, and a variety of freeware or open source solutions like XonX and XFree86 4.x if you can live with rooted X11 at the moment ( people are working on getting it to run rootless ).
In 25 words or less the server version is a MacOS X based version of AppleShareIP 6.3.x, plus a NetInfo Domain Root Server, plus a WebObjects Deployment platform, plus a Quicktime Streaming Server Deployment platform. OK thats not 25 words for those of you paying attention but you get the idea.
QTSS runs on some other UNIX OS as well ( linux + ??? ), and WebObjects deploys on Solaris and NT/2000 in addition to MacOS X.
Oh, and the Apache build in the desktop version is pretty vanilla, compared to the Server version, but you can build in whatever you like really.
Basically as a play box with a nice gui and consumer space apps that can also do UNIX stuff its pretty cool out of the box.