The OS reference page!
Linux Distro's! lots and lots of distro's- ibiblio selflessly hosts either the actual distros or link to them. Remember- the more you use linux or other opensource OS's the less ole Bill makes!
Oh- you need the dead tree to go along with those? Try the Linux documentation project
Ok- you got the OS- but now need some fun stuff to run on it? Sourceforge 'nuff said
You might think that Linux is my OS of choice- guess again. My choice is stable, time-tested, and drives the script kiddies up a wall. Yes..that's right..its none other than:

Fist- git yerself some hardware for it. Its amazingly cheap these days.
What you want something a little newer? OK- try this one
Now- go get the software. Yes- you have to pay for it but its worth it.
Not sure if you can learn it? VISTech has selflessly put up an Alpha and a Vax for free test accounts. Info Here:
Vax-centric Humor
VaxBar - if only I could
convince my wife.....
I want your vax
Vaxtrek [episodes] [movies]
The Vaxorcist
Fun with VMS
Site initiated telnet coming soon!

Sun Solaris
While the Vax is great to bang out code and do many other functions on, the demands of the modern business world unfortunately are a little too taxing for it. My main workhorse for doing the daily grind is Solaris 8 on a SparcStation 5/170.
The nice thing about Sparc's is that they are relatively inexpensive, but are incredibly powerful and reliable. This is the key word here. There is nothing more frustrating than working on a spreadsheet or a document only to have it crash in the middle of saving. The added advantage to Solaris for Sparc is security. Since most viri are aimed at M$ OS's, Solaris users can work all day/night long with little fear of some twit they offended on IRC or Usenet is going to hack into their box and render it useless.
Since my daytime work environment is M$ centric, OpenOffice (formerly StarOffice) provides the functionality of M$ Office, portability to those formats and all at a fraction of the price..
Ebay is a great place to pick up SunSparc equipment as well as software. Other sources include your local used computer shop. The other major bit of spiffiness is that the monitors for Sparcs are some of the best around in both function as well as style and because of their non-PC standard 13w3 connector, tend to go fairly cheap compared to similarly sized PC monitors. When was the last time you picked up a nearly new 20" flatscreen for $35?
Sun also is the equal of IBM when it comes to documentation. Sun.com is where you go to get started.
Other OS's to keep money out of the BOR's hands

Plan 9 from Bell Labs
I'm not really sure where to start with this one. Plan 9 is another *nix variant that is opensourse and very very stable. Take a look and see if its right for you.
I first started mucking about with CP/M back when the accounting firm my dad worked for used Xerox 820 computers. We won't talk about how long ago that was.
While I was working for a buddy of mine in his slightly more than used computer store, I happened along a NEC PC-8500 laptop. $5 and 4 "C" cell batteries later, it was mine. The great thing about this OS is that its tiny. I mean we're talking an OS the size of Window printer drivers. My 8500 has its stock 32K of memory, half of which is devoted to the OS and applications. Built in, it has Wordstar-to-go, SuperCalc-to-go, a filer (address book) and a communications package. Studley (as the 8500 is now known) is my take it to the lake to crank out some rants and a little code machine. While it not having a color screen (or a backlight for that matter) is a bit of a bummer, the 40+ hours of running time more than makes up for it. Again, I don't have to worry about script kiddies gaining access and messing it up with malicious code.
There are loads of resources on the web for it and having a working knowledge of an old OS like this is sometimes a good resume builder. CP/M based machines also make great terminals / conversation pieces.