This has to take the honors in the coolest device sweepstakes. Berlin-based 4S newcom GmbH has created iBlue, a tiny IP-PBX, that comes on an Apple iPod Shuffle.
It has to be the smallest smallest IP PBX. (of course they could have put it on a USB drive, but would not have received any attention.)
The iBlue is based on the 4S IP PBX, a software-based IP PBX. It uses SIP for VoIP and runs on a Mac mini. The device boots off the iPod shuffle, and basically turns the Mac Mini into a PBX. I am guessing it could work on any old Mac that can support OS-X. You can install the software on a Mac as well. The 3000 Euro version of the package (including Mac Mini and iPod shuffle) can support 250 users and 30 parallel calls. I guess, if they offered a smaller, cheaper DIY package for say $100, they might get more takers.
The system provides a slick and intuitive configuration front-end, which is purely web-based and accessed by simply pointing a web browser to the IP address of the IP PBX. With just a few clicks, the system administrator can configure inbound number blocks, users, extensions, outbound routing and advanced features such as auto attendant, voicemail, conferencing etc.