This has got to be the best online resource I've found for commands. You should, if you have the cache (err...cash), go out and buy the O'Reilly books Running Linux
--good for more indepth stuff and Linux in a Nutshell
--more of a desktop reference book for command look ups and refreshers.
Here's the O'Reilly's Alphabetical Directory of Linux Commands
. This is essentially Linux in a Nutshell