Linux And OSS

file_management

desktop_use

users_and_groups

system_administration

apache_webserver

php

mysql

control_panels

DNS Server bind

mail_services through Postfix, Dovecot, SpamAssassin

proftp

imp_and_the_horde

web_statistics through AWStats and Webalizer

backups Or Backing Up the Server through a bash shell script and various cli utilities

subversion (SVN)

git (version control system)

scripts shell scripts (e.g. for automating sysop tasks)

host and website monitoring tools and scripts for monitoring a linux system and its websites

virtualization mostly about small scale virtualization, specifically VirtualBox

vim the (in)famous text editor

desktop - mainly Ubuntu specific stuff

multimedia - conversion of files, codecs