Red Hat Enterprise Linux is a great distribution, but it's now geared more toward commercial enterprise, although anyone can still use it. The thing is, you have to pay for a subscription to get updates. There are some free versions of the older Red Hat releases, but they are no longer supported. Another option would be to use CentOS which is the same thing as RHEL, but without the Red Hat logos and artwork, and no paid support channel.
Mandriva, OpenSUSE, Ubuntu, and Linux Mint are also considered good distros for new Linux users.