I've been dabbling with Linux, on an old PC, for a couple of years. It is indeed free (or most versions are).
The one I get on best with is Fedora. I paid something, not a lot, for the CDs of it. I think I could have downloaded it for free, but for the size of the download it wasn't worth the faffing.
I've also installed Debian (excellent, but on my machine it's not as stable as Fedora), SuSe v 10.1 (OK), Ubuntu (bit restricted), and Knoppix (really a "run from CD" package).
The enormous plus of Debian is Kpackage - an installer that will download a complete list of all software available for Debian, and install what you choose for you. It's quite amazing what's available for Debian! The installation process is pretty much fully automatic - all that nonsense of compiling, building, testing, etc before installing is gone.
There is a similar package for Fedora, called Yum (or Yumex if you want a GUI version), but mine doesn't show anything like as much stuff as does Kpackage.
With Linux and Grub (or any of the alternatives) you can have as many versions (aka distros) of Linux as you like (and as you have hard drive partitions). You can boot to whichever one you want to use today. If you put your mail etc folders on a separate partition, you can use the same mail/data folders regardless of which distro you are running.
Once Win XP ceases to do the job, and Vista tries to make those silly restrictions, I'll be a Linux bunny.