Any Linux users here?

[Pilsner]

+1 for CentOS.

http://www.centos.org/

It's enterprise level, it's server, and it's free. It's based on RHEL, which is a very common distribution for a range of servers. If you're learning linux for IT reasons, I imagine you're going to be learning on RHEL.

OpenSUSE is another option if you're interested in working with client side stuff, though I don't see this as particularly applicable as any workstation in production that you're going to be dealing with is almost certainly going to be some strain of Windows.