Download sites for (mostly) free Unixes.
SCO has released source code and some binary images for Ancient Unixes (5th-7th editions, etc.). The binary images are particularly useful in conjunction with the PDP11 simulator available from our emulators page
FreeBSD, OpenBSD and NetBSD. We will step out on a limb here and suggest any of these to the newcomer to Unix on the personal computer. Each of these BSD-based distributions is mature, secure and reliable.
You already know all about it, here's where to get it if you want it: Linux.
As a "free Unix clone that is available with all the source code", it predates (and has a rather strange and twisted history with) Linux. Unlike Linux, it's possible for a person to study and understand the entire system within a reasonable timeframe. Yes, we're talkin' 'bout Minix.
While not, strictly speaking, a Unix variant, Plan 9 generates a lot of interest in the Unix community. Plan 9 is what UnixPorting would be running if all Unix-based operating systems were to suddenly disappear.
Solaris 10 is available from Sun, for both the Intel and SPARC platforms.