Hello! I'm a mainframe system programmer with a love for networking software and I've only installed one distribution - Debian, but I love it.
There are over 10,000 packages available, tons of online documentation (on the host Debian machine and on the internet, including a whole repository of HOWTO cookbook guides) and it's all free.
Any time I get stuck I just formulate a nice search argument, google it and get a couple screens of hits to get me on the right track or flat answer the exact question.
There is rarely something I need to ask about that someone else hasn't already asked. If all else fails there's always the source code to look at.
Here are some nice links to learn more:
(home page for Debian)
(nice Tutorial - examples of setting up Debian Linux as a server)
Good luck and have fun! Cheers!