CONV. - Convert YouTube video to MP3, GIF or MP4
Video DownloadHelper - Browser extension to download videos from the Web
Downie - Video download for OS X. Downie supports at this moment over 1,000 different sites.
FastestTube - Youtube fastesttube downloader will make your internet expirience faster harder beter stronger.How to merge video and audio .
Yout - YouTube to Mp3 Converter
FlashGot - FlashGot is a browser extension for Mozilla Firefox that is designed to make it easy to download a whole batch of files in one go. The extension is open source and free to use, but it is no longer compatible with the most recent version of Firefox.