On the Mac, I use "YTD", the "YouTube Downloader". Be careful downloading it though; some sites that had it, seemed like 'spam' sites. I found a good copy a while back (year+ ago), don't know of the best link today.
Its primary purpose is to download MP4s from YouTube videos, which it can then convert to MP3 audio-only. I find it's great for grabbing videos that I want to keep (in case someday they "go away" or are deleted). And for making MP3s, although that is is 2-step process.
Genos / Tyros5 / HK Lucas Nano 600 / FTB Maxx 40a / EV ZX1A / Rock'n'Roller cart / Hauptwerk virtual pipe organ / misc other audio & music toys