Knopsterisk
There is no end of cool things you can do with Linux. I've been following the Asterisk project for a while now. This open-source project lets you build your own PBX on a Linux distribution. Now it's even easier to do with Knopsterisk. This is a version of Asterisk that has been configured to work with Knoppix, which is a distribution of Linux that is contained on a single CD. You boot any 'x86 machine from that CD, and you're running Linux without any installation involved. Boot up the Knopsterisk CD, and your PBX is up and running in minutes with minimal installation and configuration.
The only drawback I see is it appears that the CD is only available to purchase. I can't find a download link for it. However, since it's only $9.95, it's not a huge deal breaker.