It'd be great if the mods made this a sticky.....
Many a thread has been started about what Linux distro to use. And the answer will usually be the same: Mandrake. Everyone has their favourites though, but Mandrake seems to be the most popular.
I like RedHat myself, even though I don't use Linux at the moment, but then again I'm a dork and like it when distros find all my hardware automatically and include cool screensavers.
If you want to know the best place to get ISO's,
www.linuxiso.org can help. Also, check the homepage if the distro in question.
Recap of most common distributions:
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Knoppix: Runs soley from the CD and you can't save documents.
Mandrake: Extremely popular, easy to use, comes with lots of software
RedHat: Now costs money, meant for corporate users and servers
The Fedora Project: Started by ex-RedHat employees. Similar to RH, but free
Debian: A lot of CD's, good reliability (I've never used it though)
Slackware: Not the best for newbies, many things are done through the CLI.
SuSE: Another great, easy-to-use distro. Downloading process is strange
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Browse around LinuxISO.org and take a peek at what you might like. If you are just starting out, Mandrake is the way to go. You can find it at
www.mandrake-linux.com . Just remember, Linux is free so try them all out, it only takes time (and CD-Rs).
http://www.justlinux.com/ has a forum where you can get help, but so does TechIMO.
I'll be adding more things to this thread when they come up.
Hope it helps!