Mac only: If you're not a big fan of using the Terminal but love hidden features in OS X, Secrets is the preference pane for you.
Secrets provides a list of hidden features (and descriptions of each) that can be easily toggled on and off with a simple click of a checkbox. It includes settings for about 40 different OS X applications as well, including third-party applications. The utility has just been updated to work with Snow Leopard, no doubt with some new hidden tweaks for all you early upgraders to try out.
Secrets is a free download, Mac OS X 10.5+ only.