So I've had my little monotron for four days less than a year now, and I thought I'd share a few of the neat things it can do, other than the advertised stuff. In no particular order:
Radio-style band pass filtering
Low pass filtering
Make almost any synth sound like an MS-20
Useful sound effects
Teach basic analogue synthesis
FM-controlled complex filtering of additive waveforms morphing into sine waves (with just the move of a knob!)
Be an emergency flashlight
Detuned "supersaw"-like waveforms
Make that nerdy redhead like you more (true story)
Triple oscillator detuned supersaw and sine wave.
Portable dubstep machine
A mini "toy piano"
In general, it's a small supersynth and analogue multi-effects processor which is fun to play and cheap to buy.
Note: all of these are possible if you REALLY know how to use your monotron.
'Nother note: this only applies to the original monotron, not the duo or delay, nor any modifications. Those can do different things due to their nature. Though, if you have all three monotrons, you're in business.
And people wonder why I stress actually learning your instrument... you learn its idiosyncrasies that make it awesome! I also learned my DX7. Just try to fathom what that can do...