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2Conv is free. I often use it to download YouTube videos to MP3, MP4, and AVI. If you want more formats or would like to download videos from other online sites such as Facebook, Twitch, Hulu, etc. you might try a screen recorder with audio Mac and PC. That's what I use. It helps me record videos in 1080p without latency and watermarks. I like both of them.
I have been using Any Video Converter for some time now with faultless results.
Free download here:
Easy to use, just cut and paste the URL into the window, or any number of URLs, and when they are all loaded choose from the drop-down menu how you want it saved and then click the Convert Now button. The drop-down menu has various symbols but the music note one is for sound only and clicking on that you just select mp3. Converting videos to mp3 is quite fast. However if you use it to copy videos as movies - mp4 or similar - and have a few URLs loaded it might slow down your computers other functions for a couple of hours. In that case I load the URLs and then run 'Convert Now' over night.
youtube-dl (https://youtube-dl.org) Best and reliable.
@Archtop Eddy & @BluesBop Harry I tried your links and neither worked. I did a google search and found similar names but with either .cc and .us domains. Are either of those the ones you use. DLing audio from youtube is a desirable thing and often there are nefarious entities out there with a lot of bloatware or spyware that get added to your machine. Don't want to just pick a random one.
Yes, not only that, all these sketchy ad-ridden converter sites simply use youtube-dl under the hood. Best to get it from the source... though admittedly the need to use the command line would scare many off.
There is a GUI for it though, if you're a windows user:
Technically the GUI will run on a mac, but then you have to do command line stuff, again which defeats the purpose :/
Edit: for mac, this looks good to me: