BSD

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 for your Mac
How to install NetBSD Unix on 68K Macintosh.
http://users.erols.com/ewinkler/

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/