m0n0wall is a minimal router distro based on FreeBSD, by Manuel Kasper. It is targetted at embedded PCs such as those produced by Soekris, or the WRAP platform by PCEngines. It does not work with dialup -- you need an always on connection like Cable or DSL.
There's also a m0n0BSD, which is meant for regular PCs. It requires 64 MB RAM, two NICs, CDROM and either floppy, HardDisk, or bootable Compact Flash card in an IDE adapter.
Check out CraigFalconer's m0n0wall Screenshots
A bigger version of m0n0wall is pfSense