MxTube
MxTube lets you download youtube videos. You download the videos and watch them even if you're offline. The app is pretty easy to use so you can figure that out. Its a free app, and its really useful. Search up MxTube on Cydia. It works on iOS 3.x and 4.x
Screen shots from http://cydia.saurik.com/package/com.mxweas.mxtube
