Another few good ones to know are:

df
gives available space on each disk slice. It is especially useful when used with the -h switch which stands for human readable. Example df -h

ps -ef
give plenty of info on running processes including the owner

ifconfig -a
is the same as ipconfig on a windows box

grep
this is a great command to become familiar with. It allows powerful searching

locate FILENAME
this is a simple search method on RH. less robust than find

ntsysv
this brings you to the setup screen where you can enable and disable services

man COMMAND
this gives you the manual pages for a particular command. Example man df

Well, these are some of the commands that I use in addtion to the one's already listed. Hope these help some of you out.