- CentOS (Essentially RedHat but with free updates)
- Fedora (RedHat development linux build before releasing to enterprise)
- Ubuntu Server
- Novell openSUSE (as close to SuSE enterprise as you can get without paying as far as I know).
That covers most mainstream enterprise Linux server environments. Personally, I would recommend starting with CentOS since RedHat has the largest enterprise market share across Linux distros. Once you know a single distro real well it's not hard to move across other distros. It's as easy as reading a package manager man
page to learn how to add/remove software.
Check these stats out http://w3techs.com/technologies/deta...-linux/all/all
If you do look at those stats be mindful that they are just analyzing web servers and not enterprise environments per se. So everyone and their grandmother could be running a simple non-enterprise web server which contribute to those numbers. Enterprise Linux servers are used for so much more than web alone so it's difficult to gauge which company has the largest market share. It is generally accepted that company to be RedHat/CentOS though and next to it Debian.