NetBSD for your Mac - How to install NetBSD Unix on 68K Macintosh. - http://users.erols.com/ewinkler/
MacBSD - A free UNIX variant for the Macintosh maintained by Matt Slot. - http://www.macbsd.com/
MachTen - Based on BSD4.4 and the Mach kernel, MachTen brings the power of UNIX to your desktop at an extremely attractive price point. - http://www.tenon.com/products/machten/
NetBSD/mac68k - A site that is provided as a supplement to the official NetBSD webpages. - http://www.macbsd.com/macbsd
NetBSD/macppc - NetBSD ported to the Macintosh PowerPC platform. Supports machines with a PowerPC 604 or above and a PCI architecture. - http://www.netbsd.org/Ports/macppc/