This is not a new topic. But I have recently been looking for a simpler way to download Youtube videos I have bookmarked over the past (and currently growing rapidly). There are very useful tools already created including many many plugins, etc. I also came across mps-youtube and youtube-dl.
It is surprising how easy it is to write your own script using many opensource libraries already available. mps-youtube is based on Pafy. So I wrote my own python script to download the best quality videos from a list of youtube urls in a plain text file.