A few months ago my co-located Solaris server was hit by a root kit that setup a IRC bot. It appeared that it got it by attacking one of the off the shelf web apps I use.
To prevent having to do a complete rebuild in the future if this happens again, I decided to put each major externally visible service in a Solaris Container (also known as a zone). So, I have a Mail zone, and a web zone, and then actually several more web zones that were proxied behind the first web zone. The global zone uses ipnat to port forward to the mail zone and web zone.
Then, when it turned out that the server was losing a hard-drive when I bought a new server, I was able to copy the zones to the new machine without having to re-install everything.
If I ever move away from Solaris/SPARC, I would probably do a similar setup with VirtualBox or VMWare, but Solaris is particularly nice in that patch management is unified across zones, and I believe the Copy-On-Write nature of ZFS makes for more efficient disk utilization. On the other hand, SATA drive in a modern PC mean that you probably don’t care about those features as much as you do when using a 73gig SCSI disk.