Loads that you can use on a PC. Once you've downloaded the video, you can then copy it to your phone/tablet. I forget what I use, but this may help. It's a firefox add on, so it adds a button to your FF skin, click a button and the video downloads. https://addons.mozilla.org/en-US/firefox/addon/youtube-video-and-audio-dow/
I use Keepvid.com. You copy the youtube video url and paste into the keepvid search box. It gives you various formats to save in, even just save the audio as mp3. Then copy the resultant vid into your phone or whatever device.
I use an app on my ipdad called myTube - works just like youTube on the same links but you have an option to save to cache so you can download and watch later - removes any buffering issues as well if you have a poor connection, as allowing you to take the video on a plane