Youtube Video and Audio Downloader - Download YouTube videos in all available formats and extract the original audio files.
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!
KeepVid - A free web application that allows you to download videos
YTmp3.net - Download your favorite YouTube videos as mp3 files without registration.
yt-dlp - A youtube-dl fork with additional features and fixes.
OnlineVideoConverter.com - OnlineVideoConverter.