VideoLock is an amazing, yet simple, Cydia tweak that I myself have wanted to have for a long time. I believe that Apple should have added some sort of ability to lock your iDevice but still play the audio. There is a ton of stuff I like to listen to, movies on my iPod included where I don't might just listening in. If I have seen a movie or a TV show before, sometimes it nice just to have the audio play in the background while your mind remembers the picture. Anyways, thats just me.
With this tweak, you can use it to play YouTube audio, and still have your device locked. Your screen is off and saving battery, while you can still play the audio of any music / remixes or anything else in video form. Great for finding that song and playing it in your car or anywhere else. Check out VideoLock for $.99 in Cydia.