Web: Peggo is a simple web service that takes your favourite YouTube videos, whether they're music, podcasts or other programming, and saves them as audio files so you can listen to them offline. It even removes silence to keep file sizes small, and it normalises volume so you don't get startled if you're listening through headphones.
Peggo is the spiritual successor to previously mentioned Dirpy, which is long gone but not forgotten. Using the service is simple: just paste in a YouTube video you want audio from, select the time-range you want to record (by default, it's set to the entire video), and toggle options such as volume normalisation and silence removal. Edit the title and artist if you choose, and click "record mp3". When it's finished, you'll be able to download it. Hit the link below to try it out.