I have two TiVo secret codes to share, one that I use regularly, and one that I’m about to try.
The first one is Select-Play-Select-3-0-Select. (Enter this during a recorded show for best results.) If you entered the code correctly, the box will ding three times. This code changes the Skip to End button into a 30-second skip. Really, I can’t live without this one.
The second code is Slow-0-Record-Thumbs Up. This enables you to change sort modes for your recorded programs: by date recorded (the default), by expiration date, and alphabetically. The nice thing is that you can switch sort options on the fly, just by using the 1, 2, or 3 keys on your remote. Anyway, this one should be useful if you have a lot of shows to sort through.
You can find both of these codes at DBSTalk, and a more-thorough (but harder to read) list is available at the TiVo Community Forum. Try them out when you get a chance. Those TiVo guys really think of everything!