I have noticed that the overwhelming majority of visitors come to this page via a Lycos search. This page is probably *not* what you're looking for - see the links at the bottom of this page for more useful information!
jobs --- lists your currently active jobs (those that you put in the background) and their job numbers. Useful to determine which one you want to foreground if you have lots of them.
bg --- background a job after suspending it.
setenv --- lets you set environment variables. For example, if you typed a wrong value for the TERM variable when logging in, you don't have to log out and start over, but you can just do setenv TERM vt100 (or whatever). To see what all your environment variables are set to, type env. The one that you're most likely to have to set is the DISPLAY variable, when using an X-display.
unset VAR --- lets you un-set environment variables. Useful, for example, if you've usually set autologout but want to stay logged on for a while without typing for some reason, or if you set the DISPLAY variable automatically but want to avoid opening windows for some reason.
ispell filename --- will check the spelling in your file. If you're running it on a LaTeX file use the -T option to tell it to ignore the LaTeX commands. You can create and use your own dictionary to avoid having it tell you that your own name, those of fellow linguists, and linguistics terminology are a typos in every paper you write.
CTRL-p --- show the last command typed, then the one before that, etc.
| (piping) --- Lets you execute any number of commands in a sequence.
man ls | grep time --- checks whether the man page for ls has something to say about listing files by time - very useful when you have a suspicion some command may be capable of doing what you want, but you aren't sure.
ls -lR | grep dvi --- will show you all your dvi files - useful to solve disk space problems, since they're large and usually can be deleted.