MINIX is a free UNIX clone that is available with all the source code. Due to its small size, microkernel-based design, and ample documentation, it is well suited to people who want to run a UNIX-like system on their personal computer and learn about how such systems work inside. It is possible for a person unfamiliar with operating system internals to understand nearly the entire system after just a few months use and study.
normr's firewall/router is another single floppy distribution of Linux. This distribution lets you use an old 486 computer to share a cable modem or ADSL connection. A Web-based interface makes this distribution simple to setup and maintain.