CONV. - Convert YouTube video to MP3, GIF or MP4
FastestTube - Youtube fastesttube downloader will make your internet expirience faster harder beter stronger.How to merge video and audio .
Downie - Video download for OS X. Downie supports at this moment over 1,000 different sites.
Video DownloadHelper - Browser extension to download videos from the Web
Yout - YouTube to Mp3 Converter
GrabAnyMedia - Grab Any Media is a popular video downloading extension for the web browsers.