My friend told me of this website, it is completely free and legal to download a video, all you need to do is paste a link to a video into the box and it brings up a number of options and formats for the video to be downloaded in. Here are the terms: http://keepvid.com/terms
The website itself is legal. How you use it may or may not be legal. If you use it to save files for your own personal use that are not protected under copyright law, or for which the copyright holder allows free distribution of the material, then you are in the clear. If you use the website to make copies of someone's video to distribute for commercial purposes, then you may be violating copyright law.