This website has great resources for people just getting started with Linux / Unix. It explains most common Linux / Unix commands in simpler, more human terms than the man pages do. This one is espically good if you’re transitioning to Linux / Unix from Windows / DOS.
This website has very little as far as original content goes, but it does have all the Linux man pages online which can be quite useful at times.
Tlpd.org always seems to have great articles about almost anything Linux related, and basic shell usage is no exception.