Mac OS X Leopard only: Free, open source application Secrets rolls every hidden feature of Leopard into an easy-to-use preference pane in your System Preferences. Developed by the author of the much-beloved Quicksilver, you can browse Secrets by application to see what tweaks are available for the apps you have installed. There's a ton to look into with Secrets, but the one secret that blew my mind off the bat was the "Arrows link to library instead of store" tweak in iTunes, which may be the most useful thing I've ever done to iTunes (and I've covered the best iTunes add-ons). Secrets is free, Mac OS X only. If you give it a try, share your favourite Secrets in the comments.