I have been looking for a good, easy to navigate, yet extensive UNIX reference. I finally found one here: http://wks.uts.ohio-state.edu/
I must say that I did not find this myself - it was recommended on a MacNN forum that I started discussing creating shell scripts.
[Editor's note: I've also added the link to the Links collection here for easy future access.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20011113190742846