iPodLinux is an open source venture into porting Linux onto the iPod. So far, it have successfully ported a customized uClinux kernel to the iPod, and written a simple user interface for it dubbed podzilla. Additional applications and modules have been written, adding many capabilities not found in Apple's firmware.
iPodLinux is currently safe to install on 1st, 2nd, and 3rd generation iPods. Development is currently on-going on later generations of iPod, including the fourth generation click wheel, mini, U2, Photo/Color, Nano, and Video.
The installer is currently in development for Linux, Windows and Mac OS X. It will only work on some iPods. Check Project Status page to see if your iPod is supported; the "improved" revision of 5th Generation iPods, and presumably any iPod running an Apple Firmware newer than Sept. 12, 2006 (iTunes 7), will definitely not work. The installer is written using Qt.
The installer was made possible by the rawpod library; it basically throws its own device system on top of the raw iPod device, so it can partition, format, read and write files to the ext2 partition, etc.
The installer manages your installation as a set of "packages"; each one is just a tar.gz archive that gets extracted into the iPodLinux partition, but the installer remembers what packages are installed and what files they contain, so you can remove and upgrade packages easily. A package might be the userland, a podzilla, a pz2 module, iBoy, etc; pretty much anything that works on iPodLinux.
Download here installer iPod Linux from Windows, Linux, and Mac OS X.