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.
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.
ClipGrab - A friendly downloader for YouTube and other sites
OnlineVideoConverter.com - OnlineVideoConverter.