Android: Previously mentioned Peggo has always been a great tool to grab the audio from YouTube videos or Soundcloud songs so you can listen to it offline, like a podcast. Now the app is bringing its features to Android so you can listen on the go.
Like its desktop cousin, the Android version of Peggo can also normalize volume (so you don't blow out your ears when someone's super loud and someone else is quiet, you know those podcasts) and can remove silences to trim down the file size and save you from having to listen to long pauses or breaks in your recorded track.
The app also supports YouTube and Soundcloud, which is super useful if you have a long interview or program on YouTube you'd rather just listen to while you commute, or if your favourite podcast publishes to Soundcloud but doesn't let you download episodes to listen to offline, on a flight for example. Everything you save offline is saved at its highest available bitrate too, so you don't have to worry about losing quality just to listen offline.
Peggo for Android is completely free. It's not available at Google Play, but it is available in the Amazon App Store, or you can grab the APK and sideload it yourself. In every case, links to download at on the homepage below.