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.