Backup Brain: What's your favorite Mac OS X utility?

Tom Negrino, over at Backup Brain, is working on a new book on upgrading to Tiger and asks his readership for suggestions for favorite Mac OS X utilities: "So what are your favorite Mac OS X utilities and other add-ons? What makes your life easier? All sorts of system enhancements are welcome, so if you have a favorite widget, font, or anything else what doesn't come stock with Mac OS X, please let me know about it in the comments."

Feel free to share your responses in the comments here, as I'm curious to know, but make sure you head on over and lend Tom a hand as well.