Platform: Android, IOS
A chord is one of the first things any beginner has to learn and this is a good tool for understanding how songs are written. Irrespective of your experience with song writing, Chords Progression Reference is sure to pique your curiosity.
While it isn't the best looking app out there, it has a minimalistic design. All you have to do is select the key you want your progression to be in and then pick from major (happy sound) or minor (sad sound) and you're done. The application lists down all possible chord progressions within those parameters and you can listen to each progression. You can also see each individual note that are being played and also switch around chords to see whether they fit. If you're just starting to learn the guitar, this is a great tool to have access to early on. It will not only help you expand your horizons, but it will also give you an idea about chords you might not know about.
Despite being an app geared towards a musician centric audience, if you have any interest in music in general then Chords Progression Reference will give you greater insight into how your favourite tunes are written.