Free Video Catcher is an easy-to-use tool which aims to automatically download online videos as you watch them.
There's no messing around copying URLs from your browser, then, or otherwise switching between applications. Instead, once Free Video Catcher is running, it monitors your internet connection looking for videos, and captures a local copy of anything you might watch.
The authors say the program works on all the main video sharing sites (Youtube, Dailymotion, Megavideo, Vimeo, Google Videos, MySpace, veoh, Songza, LastFM, Jiwa...), and that was our experience. We had no problem capturing videos, and these could then be launched from within Video Catcher, or you might open its temporary video folder to access the files as normal.
And if you're only interested in music then you can switch to an "Audio" mode, and have the program only capture the soundtrack of each video.
One small down side with this approach is that the program will probably capture more videos than you intend, or really need. And this may also be an issue if you're a little short on bandwidth. We switched quickly across a series of videos on Youtube and Free Video Catcher wanted to download all of them, in full.
If you're careful when you use Free Video Catcher, though, this shouldn't be a problem. Simply leave the program off when you're casually browsing, turn it on when you know you want to capture more interesting videos, and it should save you a great deal of downloading time.