I download a bunch of videos from Youtube using the great Minitube app which you can find in the Ubuntu Software Center. I transfer a lot of documentaries onto a USB stick and then plug into my TV to watch when I have a free moment. I also download music videos from my favorite artists, put them onto a stick and run it on my TV while doing chores around the house.
There are some videos however that are blocked from downloading in Minitube. Especially the VEVO videos. I've found a great little program called 4K Video Downloader which downloads the VEVO videos perfectly. They have Ubuntu DEB files for 32 bit and 64 bit and the app is cross platform so you can also download it for Mac and Windows too.
The 4K Download site also has a few other programs you might be interested in. Youtube to MP3, Video to MP3, Stogram which downloads Instagram photos, and a couple more apps. Possibilities are endless. I like the idea of downloading video to MP3 especially if it is a talk, self help video or how-to video. I can always put the file on a USB stick and plug it into my car radio and listen on the way to work!