Bookmarklet
Drag this button onto your Bookmarks toolbar: Converto.io DL
Click Converto.io DL while watching a video on YouTube to download it as MP3 or MP4
Alternative way to download from YouTube
While watching a video on YouTube:
Replace "youtube.com" with "converto.io" in the URL
Press 
Enter
Choose conversion format to convert YouTube video to MP3, MP4 or GIF