This is a thing that brought me to the real world. As far as I remember my first fully configured service was ProFTPD. But when we talk about system administration the only one thing comes to mind: operating system itself.
Personally I play with two UNIX-like OSes: OpenBSD and Linux. While OpenBSD is being a fully functional OS, Linux is just a kernel and has a lot of distributions which contain it and also userland (programs).
I have my own preference here. After more than 5 years of research, I've stopped at SMGL (Source Mage GNU/Linux).