I've tried Fedora, Sabayon (Gentoo based), Ubuntu, SUSE, Linux Mint (which I guess is the same as Ubuntu), PCLinuxOS, SAM (based on PCLinuxOS), Ubuntu Studio, Kubuntu, Xubuntu, and PuppyLinux.
My favorite by far has been Ubuntu. The forums are excellent, which is one of the reasons I've stuck with it. Also, you really can't beat Aptitude for getting software, but I guess that's just my opinion. When I tried SUSE, I found the community to be not so helpful, and YaST was a major pain. It was nowhere near as easy as aptitude was.
SAM Linux is really good when you need a distro that doesn't take up a lot of system resources. I used it on an old Pentium III laptop that was on its last leg. It was a lot better than Xubuntu. Zenwalk might be better for older systems though. I don't think I ever got around to trying it (or if I did, it didn't support my network card, so I didn't use it).
I really liked Sabayon Linux. It had every piece of software I needed. The problem was, I couldn't figure out how to get emerge (gentoo package manager) to work. Every time I tried to install something, I'd get some error about packages being locked or something (can't remember, long time ago). But, if you can get emerge to work, I imagine that it would be a great distro.
Linux Mint was a really good distro too. It's based on Ubuntu, so it has all the good stuff that Ubuntu has, and then it has a lot of the stuff that Ubuntu doesn't have enabled by default for legal reasons, which is nice.
I agree with anandtechuser07 that you should try a virtual machine to test what distro you'll want to use. Virtualbox is also useful if you need to install windows inside your linux distrobution in case there's some program you need that doesn't work in WINE, or some device you need doesn't work in Linux (for me, its my Canon MFC. I still can't get scanning to work in VirtualBox though).