General Unix Information
Unix Guru Universe
Unix Reference Desk
Don't miss the Free Unix Giveaway List. Unix users who have spare CDs sign up to give them away for the cost of postage. It's a good way to try out Unix for very low cost, although the versions offered are usually somewhat old. Still, I have received both Linux and FreeBSD distributions from this list. It's worth a look.
Linux
Information
Linux.org
Linux.com
LinuxWorld
LinuxPower
News
Linux Today (sure, I know that Slashdot is better known. It used to be the Linux news source. Not any more. Linux Today simply has more content.)
Linux Gazette -- online magazine
Linux Distributions
Caldera's OpenLinux -- a good distribution for beginners
Red Hat -- a very popular distribution
Mandrake -- another good distribution for beginners
Debian and Slackware -- distributions for more advanced computer users
Cheap Linux
Linux System Labs
Linux Central
FreeBSD
General Information and News
The FreeBSD Project
FreeBSD 'zine -- online magazine
Daemon News -- another online magazine
Getting FreeBSD
FreeBSD doesn't come in the variety of distributions you find in Linux. Instead, you can find an official 4-CD set at Walnut Creek, or a get cheap 1-CD version at Linux System Labs. Recently FreeBSD has been showing up at CompUSA and Borders Books in the form of The Complete FreeBSD and FreeBSD Power Pak. If you want to use FreeBSD, I highly recommend one or the other of these. The book is a great help on many subjects and is the only easily available FreeBSD book in print in English.
Solaris
You can find a "free" version (you pay for the cost of media and shipping) of Solaris 7 at Sun Microsystems. My copy ended up costing around $18.
SCO Products
SCO OpenServer and UnixWare 7 are available from SCO's web site here. They too are available for the cost of media and shipping, which works out to more than $50 for OpenServer and $20 (without manuals) or $49 (with manuals) for UnixWare 7. My personal opinion is that you're better off with Linux or FreeBSD than with Solaris or SCO, but these are available if you want to try something different.