The underlying open source basis of OS X includes a whole smörgåsbord of enterprise-strength Unix server tools – Samba for Windows networking, Apache, Perl, Python, Ruby, Rails and MySQL for Web hosting. There’s CUPS for printer management, Dovecot for POP3 and IMAP e-mail delivery, Postfix for SMTP, SquirrelMail for webmail, Mailman for running mailing lists, SpamAssassin for junk filtering, and so on down a long list that you can peruse here.
Apple's own tools include iCal Server 2 for CalDAV-compatible calendaring, Address Book Server for CardDAV-compatible shared address book and LDAP directory, a Wiki server, QuickTime streaming media servers and so on. Obviously everything is compatible with Apple clients – Macs and iOS devices – but it's also a highly Windows-compatible server. It can even act as a domain controller, if you wish, or join an ActiveDirectory domain.
If you want to make the machine Internet-facing, it can act as a VPN endpoint or even, in theory, as a firewall using a USB Ethernet adaptor. All these services are controlled by some remarkably simple and friendly GUI management tools and the machine's desktop can be accessed remotely using VNC.
There are two levels of admin tool, one for novices – who don't really want to fiddle too much – and another for power users. The basic tool is Server Preferences and it looks much like System Preferences, the OS X equivalent of the classic MacOS (or Windows) Control Panel.
Server Preferences has just three groups of icons: Accounts, Services and System. Accounts lets you add and remove users and groups, System contains various monitoring tools, and in between them, the row of icons under the heading of Services lets you do the bare essentials of administration in the various roles that OS X Server performs.
Each service can be turned on or off, you can create, remove or modify shared resources and so on. There's very little in the way of customisation – this is really just for the network admin who wants to plug the box in, turn it on and leave it.

The Server Administrator gives an overview of what's going on
The centrepiece of the set of more sophisticated tools for advanced administration is called Server Admin. This not only lets you drill down into all manner of settings and twiddle options to your heart's content, it can also control multiple OS X Servers from within a single interface. Everything – that on a standard Linux server would be accomplished by editing config files – can be done from within a friendly, polished GUI, complete with online help, and it can manage remote boxes too.
COMMENTS
Hmmm, call me a cynic, but....
This is hardly a replacement for an xserve though (http://www.theregister.co.uk/2010/11/08/apple_xserve_server_dead/).
Unless you build resiliency into the software stack (i.e. buy multiple servers), I don't see redundant nics, PSU or disks, let alone having a pretty weak cpu.
As a soho server for less than a dozen users, I can see a niche. For anything business critical, Apple have left many customers in trouble. Time to move to Windows or Linux on a Dell/HP/IBM box or similar.
The Mac mini makes a great workgroup server
We're using a Mac mini (earlier model) running OS X Server as our office server. It handles file sharing, user authentication, internal/cache DNS, calendar sharing, instant messaging, email, web proxying, intranet and wiki. Does a great job and I would thoroughly recommend this solution.
Its worth mentioning
that Server Admin, Workgroup Manager and a bunch of other apps in the suite can be installed on your desktop Mac and used to control instances of Snow Leopard Server from afar.
@E2
You have no clue what you are on about at all have you.
No health monitoring nor reporting - yup right there built into the server tools including alerting
No remote monitoring - wrong again build into the offering
Warning about pending hardware failure or redundant - sure you have got a point there but if you require that of each server than you are talking about several factors price category difference. No point comparing against such a small price item.
Management interface - sure others do get close but until you've actually used it you just have no idea how well everything works together and besides those openspurce components there are also a number of other services not available elsewhere. If you are happy tinkering, sure build it all yourself. Some people want to get on with their business
Daisy chain FireWire - I agree that seems daft, mine just uses iscsi volumes on my San. Nice flexible storage solution. I just have two cables out the back one for power and one for Ethernet. Granted. Second Ethernet cable would be nice but as a workgroup server that just doesn't matter.
Now the best bit for me is that it is silent and only uses 9.44w at idle and I've never recorded more than 18w under load. That combined with the San running on fan less d510 motherboards with 8 disks it is a very low power silent high solution.
Small Office / Home Office
If I'm a mac user, I'll use OSX server for the same reason windows desktop users tend to install windows servers - its what I know, and what I know, I can administer and that makes it cheap. Linux or windows suddenly becomes less cheap if I need to hire someone at £25k/year to administer it, or if I need to take time off income-generation to learn it.
In a soho environment, I don't have space for a 19" rack, I don't have dual power feeds and my connectivity requirements (vpn, calendar sync, time-machine etc) exceed my data capacity requirements. I also don't want server noise in my little office.
A small market perhaps, but it is still there.

