GYD - Graphical Youtube Downloader is a GUI for youtube-dl.
It is easy to use and it supports most of the youtube-dl features and some extra features like converting files and a "youtube to MP3 / OGG" (video to audio) function since Version 0.3a.
When you like to use the extra features you must install ffmpeg.
Youtube-dl - http://rg3.github.com/youtube-dl/
NOTE: GYD 0.3.x is the last version using QT4 (does not work with QT5).