Chordex places a wealth of musical chord theory at your fingertips.
It offers two modes of operation. In the first, you can select a chord and the app will list the notes in that chord, telling you both their names and their degree on the scale. You can also play the chord or individual notes on the built in tone generator.
The second mode allows you to highlight notes on a virtual keyboard, while the app lists the chords that contain those notes in an order based on both how well they match and their obscurity. For example, if you select D and F, it is fair to say that D minor is a better fit than A flat diminished 7th, but both are options. Again, this mode lists the notes that make up each of the suggested chords and offers you the chance to listen to them all.