distros I've used are: as follows.
redhat from redhat 7 through to fedora 7 and enterprie linux 4
mainly used this on webservers, DNS server, proxy servers, firewalls, mySQL servers and Oracle servers and Mail servers.
suse: used simply as a desktop to test my Uni coursework
Debian 7: used for a short while as a web server OS and as a firewall
CentOS: used as a DNS server and as a mail server
and DSL just for fun.
I'm pretty much a redhat guy through and through though when it comes to Linux distros.
(i've also used the stripped out versions of linux like the smoothwall linux distro that just runs a firewall, and I've used the stripped out version of Linux that runs on Nokia (and I assume a few others I just don't know about) firewalls and routers when setting up these devices...
In addition to that, those looking to learn linux for the server OS might also be interested to know that Suns Solaris which is a true UNIX OS has been freely available for years.
and I've used that for running LDAP.
oddly enough though, even with that experiance I'm still much more aufait with microsoft technologies.
edit - also I've never actually used Ubuntu. though I've heard good things I just think that if I wanted to use a powerful OS with the feel and ease of use of windows, I'd just keep using XP setup and secured correctly!