Music Theory
A few examples of what you can ask Wolfram|Alpha about:
compute the frequency and other properties of a musical note
specify the octave
calculate the note corresponding to a given frequency
analyze a sequence of notes
specify notes using MIDI codes
find the Parsons code of a note sequence
identify an interval
identify several intervals
get information about a chord
specify a chord using lead sheet notation
identify a chord from constituent notes
get information about a scale
generate sound with a specified waveform
superimpose multiple waveforms
get information about an instrument
find properties of an instrument