This article is from the Amiga FAQ, by Ignaz Kellerer with numerous contributions by others.
Actually there are three Unix versions on the Amiga. All of them need at least 68030 (see 68EC0xx), probably a 68040 in the near future and seem to have problems with many hard-drive-controllers. You should have a good look into the documentation before installing it. Unix needs much resources, say at least 10Mb RAM and a 150Mb Unix-partition on the hard-drive.
1. Commodore offered a System V Unix in the past. It contained TCP/IP, X11 and other software and seemed to make a good job, but it was expensive and after all Commodore has dropped developing it. Commodore-Unix needs a streamer because it is distributed on tapes.
2. A Linux port is prepared. But actually there is not very much than the kernel. Specialists might like to use it, but it cannot be recommended for now. Linux is freely distributable. Sources: `ftp.tsx-11.mit.edu' or `ftp.uni-erlangen.de', both directory `/pub/Linux/MIRROR.tsx-11/680x0'. Or `ftp.leo.org', directory `/pub/comp/os/linux/680x0'.
3. NetBSD is freely distributable too. Like Linux it isn't ready for now, but it seems to make big steps. Most GNU software is said to run, especially emacs and gcc. I think it's worth to have a look on it. Sources: `ftp.uni-erlangen.de', Directory `/pub/amiga/unix/NetBSD-regensburg'. See Fish CD.