Sometimes I have to work in two directories from the commandline. For instance to check a log file got created, then back to the directory where the command I am testing is run from. The quick tip is simply "cd -". This takes you back to the previous directory. If you type it again it takes you back to where you were, acting as a nice toggle.
"cd" with no parameters takes you to your home directory. "cd ~/somewhere" takes you directly to a directory relative to your home directory (i.e. tilde expands to your home directory). I had a hunt around but couldn't find any other special characters to use with "cd", but if you know one please let me know.
(This tip was found in Linux Magazine, June 2008)