CatchYouTube enables you to convert YouTube videos into any number of different video and audio formats and then to download these to your computer. It's FREE and very simple to use.
About MPEG Streamclip
A converter for MPEG-1/2/4, QuickTime files and transport streams (.ts and .m2t). For MPEG-2 it works together with the Apple MPEG-2 Playback Component and provides high-quality conversion of movies into several useful formats for Mac OS X. It is compatible with Apple Final Cut Pro, DVD Studio Pro, and Roxio Toast 6/7/8.