Without question, one of the best and worst features on the iPhone is YouTube. Being able to watch YouTube videos from anywhere at anytime isn't just convenient, it can be a lot of fun as well. However, YouTube on the iPhone isn't without its frustrations.
For one thing, you tend to want to show people the same video over and over. Waiting for the video to download repeatedly is less than exciting. Additionally, videos originally seen at WiFi quality (the iPhone downloads higher quality video when connected to the internet via WiFi) lose a lot of their luster when seen at EDGE quality.
The solution to both of these issues is the ability to download and save videos from YouTube directly on your iPhone for replay later. This ability and more is provided by a new 3rd party application for the iPhone called MxTube.
MxTube lets you search YouTube videos via an interface practically identical to the native YouTube app. Once you've found the video you're looking for, you can select to download either the WiFi or EDGE version and save it directly on your iPhone for countless replays.
MxTube requires a jailbroken iPhone and can be installed via Installer/AppTapp. Check out the video below for a demonstration of MxTube (ignore the "coming soon" message at the end, MxTube is available now).
More info about MxTube is available at http://mxweas.com.