This is the best download script I found yet. I’m using it for some months now, and I can say that it has always been reliable and if there was a change on youtube and it didn’t work any more, there was an update on usersripts.org a short time afterwards and it worked again.

My Tip: Right-Click and Copy the Link to open the video with VLC (or configure Firefox to use VLC for these types of Links), and then you can use all it’s features live. I used this to synchronize Videos that were on Youtube in two languages, The English one had the video I preferred (HD) but the German one had the text I wanted.