What is it? Its a virtual machine that is able to run Mac OS 8.x - 10.2 and Linux trough use of virtualisation.
Why? Because there isnt any virtual machine software for Mac OS X at the moment, and software like this is a must if you like to tinker with an operating system a lot. The PC side has had products like this (VMWare, Virtual PC for Windows) for ages.
Heck, it would even have a miriad of commercial appliances, for example customer support for older OS'es. Sure we have BasiliskII and vMac, but they leave a gap between 8.1 and 9.2.2, and dont do PPC.
Plus, wouldnt it be cool to be able to run Linux within Mac OS X without having to reboot?
License : GNU
Website : http://www.maconlinux.org/
source code : http://www.maconlinux.org/download.html