I use a YouTube Downloader quite a bit to download songs or snippets of video related to the subject I am teaching.
There are several kinds of YouTube Downloaders but I like the one found here.
We (the teachers) have access to YouTube at school however it is dicey at best to try to stream from YouTube. If there isn't enough broadband width the video will pause and try to catch up with itself. Sometimes the advertising bar along the bottom pops up with inappropriate ads for the classroom (alcohol, condoms, etc.). The YouTube Downloader eliminates this and is easy to use.
I always use it from home, since my internet is faster. Above is a screen shot of the icon on my desktop and what it looks like when I open it up. I simply copy and paste the address of the video I want to use and tell it where to copy on my computer. It will download it as an MP4 file. I can play an MP4 at school but most time I convert it to a WMV (Windows Media Video) so I can embed the video in my Promethean flip charts. To convert it you would use the same YouTube Downloader and tell it to convert the video (look at the screen shot above - under "what do you want to do?" you have two choices "download" or "convert").
Most recently I downloaded several songs from YouTube that were made up to teach the scientific method. I had taught my version of the scientific method (with fun hand gestures) andthen showed students all the other versions I had found. They had to compare my version with those from other teachers. It was great to show that students all over the country and in different grade levels are learning the scientific method.
Here are links to the songs I used: