It would seem that the Mac OS X development spectrum is bursting with excellent freeware applications. TubeTV is one such freeware application that fills a void that many Mac-YouTube users have -- saving videos in a Mac compatible format for later viewing (there are some paid alternatives out there, such as Tubesock but TubeTV is free, and offers up some additional features).
TubeTV features a built-in web browser that allows you to surf over to your favorite video sharing websites. When you are viewing the video you want to download for later, just click the little download button in the toolbar. The video will begin downloading and then covert to your specified format on-the-fly. TubeTV uses the free Perian to convert the videos to either iPod, AppleTV, iPhone, or a standard phone format. When I used it, the software was spot-on. It downloaded the video very quickly and converted a 3 minute video to iPod format in well under a minute. The great thing about TubeTV is that you aren't limited to using only YouTube, you can use it with other flash-based video sites as well. I tried it with Viddler, and it worked as advertised. The video quality is also excellent.
TubeTV can be downloaded from the developers website for $0.00 in all currencies (however, donations are accepted).