The fish shell is a modern command-line shell that aims to be more discoverable and less beholden to tradition than other shells like bash and csh. Among its many unique features are on-the-fly syntax highlighting of the command line, as well as tab completion for everything under the sun with descriptions for each possible completion. Unfortunately, this shell does not ship with Mac OS X, so you'll need to do some work to install it.
First, download and install MacPorts. This will provide a Terminal command, port, that you can run to download packages. If you prefer a nice GUI for this, though, I recommend downloading and installing Porticus as well.
If you're using plain old port, type port install fish at the command line. If you're using Porticus (or another MacPorts GUI), search for "fish" in "All Packages" and click the Install toolbar icon.
Because fish requires several large libraries that probably aren't installed yet, and must therefore be downloaded and installed before fish can be used, the installation will take quite a while. Go grab a sandwich and come back in a half hour or so.
Once the install is complete, fish is installed. You can run it from Terminal right now by typing /opt/local/bin/fish, and play around with it for a while to get a feel for it. (Hint: you can change the prompt by redefining the fish_prompt function.)
If you decide you like fish better than bash, you'll probably want to set it as your default shell. The process for doing this is not quite as straightforward as it should be, and might qualify as a hint in its own right. You can change a user's shell by opening the Accounts preference pane and right-clicking on that user's name in the list on the left, then choosing "Advanced Options...". However, if you attempt to set your shell to /opt/local/bin/fish, you will get an error when you launch Terminal claiming that the administrator has set your shell to an illegal value! To get around this, you must also add the line /opt/local/bin/fish to the file /etc/shells, which Mac OS X consults to determine what constitutes a "legal" shell. You will need to enter your password to edit the file.
[kirkmc adds: I haven't tried this.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=2008081723483256