MxTube, the iPhone’s first native youtube video downloader. Download your youtube videos and watch them later. Works on all devices 3.x and 4.x. MxTube has been existing since ages ago even before iPhone 3G released. MxTube is a free apps that available on Cydia through BigBoss repo and developed by Max Weisel Guitar.
MxTube has another version called MiTube. It was a great apps on iTunes AppStore ever, then Apple took it down.
To download youtube videos using MxTube, first, you need to jailbreak your device (see iPhone Jailbreak category on iPhoners.org). Find and install MxTube from Cydia, then tap MxTube icon from Homescreen.