I hate, hate, proprietary software.
If you spit when you hear Bill Gates on TV and think Novell is a traitor to Linux for partnering with Microsoft, then there are several Linux distributions just for you. The one I've used the most is gNewSense. A variation of gNewSense is also RMS' (Richard M. Stallman), free software's founder, preferred Linux distribution.
I'm not crazy about proprietary software, but what I really want is cutting edge Linux.
Sound like you? Then, chances are you're already using Fedora. This, Red Hat's community distribution, is both an outstanding Linux in its own right and takes Linux about as far as you can go without being a Linux kernel developer.
What's that? You want to be a Linux kernel developer? Well, the other distribution you might want to look at is Gentoo. This source-code based distribution lets you get your hands dirty with every aspect of the Linux experience. It is, in no way, shape, or form, a distribution for beginners. But, if you really want to know Linux from the inside out, it's the operating system for you. You'll also want to check out the Linux Foundation's free training Webinars to see how the real pros of Linux go about building Linux.
I've got computer troubles and I've heard Linux can help.
You've heard right. There are several Linux distributions that are designed to help you bring dead PCs back to life no matter what operating system they're running. I have two favorites in this line: Damned Small Linux, which will run on almost any 486 or newer PC, and SystemRescueCD. With both, I've brought PCs back from fried hard drives, corrupt memory, and innumerable cases of Windows malware crud. If you ever do computer repair, you must have at least one of these in your repair kit. They're incredibly powerful and useful.
OK, so what do you use?
Who? Me? After more than a decade in Linux, I use several Linux distributions on a daily basis. These include Fedora 12, openSUSE 11.2, and Ubuntu 9.10. For work-a-day desktop work I tend to stick with openSUSE and one distribution I haven't mentioned yet: MEPIS.
MEPIS is relatively unknown and that's a pity. What I like about it is that it combines ease of use and great stability. Linux systems are known for running for weeks and months without problems, but MEPIS is exceptional even by those exacting standards. I'm sure I rebooted my main desktop sometime last fall, just don't ask me which month!
In addition to those virtues, this Debian-based distribution provides a nice blend of cutting edge software with old favorites and it also includes access to the most important proprietary programs. If it wasn't such a small operation—it has only one developer—I'd recommend without reservation as a business desktop. Some day, I hope some venture capitalist will realize what a diamond in the rough MEPIS is, and give it the kind of support it needs to become a major Linux player. In the meantime, if you know some Linux and you want an outstanding distribution with a KDE interface, may I recommend you give MEPIS a try. You'll like it.
So, did I miss your situation? Drop me a note here in the comments and I'll see if I can find just the right distribution. In the meantime, I'd also like to know what distributions you've found to be your perfect fit and why.
Now, with all that being said, get on and give Linux a try. You'll be glad you did.