iPod Linux is an open source venture into porting Linux onto the iPod, it is based on uClinux's kernel.
iPod linux features additional applications which have not been written for Apple's Firmware.
It is safe to install on 1st, 2nd and 3rd generation iPods, Development is currently underway for the later iPods with click wheel, mini, U2, Photo/ Color, Nano and Video.
A list of compatible ipods can be found here
Note: Debian/Ubuntu users must install the libpng3 library (apt-get install libpng3) before attempting to run the installer as it is not installed by default.