Youtube Mp3 Today - Convert youtube videos to mp3 in 1 click every time
Video DownloadHelper - Browser extension to download videos from the Web
MediaHuman YouTube to MP3 Converter - Extracts MP3 audio from YouTube, Vimeo, Dailymotion, Mixcloud, Bandcamp, SoundCloud and many others. Available for macOS, Windows and Ubuntu/Linux. FREE!
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.
yt2mp3.org - Free online YouTube to MP3 Converter
ClipGrab - A friendly downloader for YouTube and other sites