Markus built a software AM radio transmitter using the Arduino. The audio signal is fed to the ADC input through a decoupling capacitor. A PWM output pin directly drives a capacitor inductor circuit with an antenna. The schematic and firmware are available.
Well, could have been done with one transistor, but nice idea.
But dont ask about harmonics!
Make a software defined radio receiver: AM demod = sqrt( I^2 + Q^2 )