This is one of those apps that might not be in the App Store tomorrow. MiTube lets you download MP4s(iTunes link) of YouTube videos to save on your iOS device.
MxTube has been a Cydia app for awhile but for whatever reason Apple approved it. Get it while you can.
Update: Apple called the developer, the app will be down momentarily so get it while you can (No excuses)!
Update 2x: The app has been completely pulled from the App Store. Sorry to those who missed it as the app’s awesome.