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!
Video DownloadHelper - Browser extension to download videos from the Web
YTmp3.net - Download your favorite YouTube videos as mp3 files without registration.
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.
OnlineVideoConverter.com - OnlineVideoConverter.
ClipGrab - A friendly downloader for YouTube and other sites