I use YouPipe on my phone, but I've never had any difficulty finding an online downloader by just googling "Youtube MP3 Download" or the like. Which one turns up might vary from time to time, but there's always one that works :-P

Others have also mentioned youtube-dl, which is probably the gold standard. It has a GUI (not sure if it's part of the default download - I got it via my linux distro) and has lots of options for audio only or video or whatever...