I'm fairly new to Windows development, but had a couple of years on Solaris before that.

When I got my Windows gig, one of the first things I did was find a couple of old machines and put FreeBSD on them, because I knew I would want servers and network tools either not available on Windows or very expensive.

I've learned more about Unix by hacking FreeBSD than I ever learned in a Solaris production shop.

Solaris is of course much slicker, but FreeBSD is a coherent whole (unlike GNU/Linux), and the documentation available is remarkably good.

To get the basics of Unix server administration under your fingers, you could do a lot worse than setting up your favorite servers on FreeBSD.