I believe it was squelch that commented on mandrake (preferred) and SuSE not being free. For windows users, that would like to "try out" linux without modifying their harddrive, do a search on google for "knoppix". Knoppix is a Debian Linux based distro with many customizable features. If you are a user that is wanting to install linux and dualboot your machine, I would recommend Fedora, SuSE, or Mandrake....all which mostly hold your hand through everything you do. They are still quite powerful. More experienced users like to use Debian or Gentoo linux. These last two distros are not recommended for beginners because of the way the installer looks (all text) and they both require the user to have knowledge of many of the packages being installed onto the system.
All of the above mentioned distros have free versions, while also having pay version because of the features they entail. You will find links to the most popular distributions at
www.linuxiso.org. You may also want to check out
www.distrowatch.com for reviews, comparisons, and downloads of the MANY distros of linux.