One very interesting application for the Propeller is 1-bit Sigma-Delta ADC (analog to digital conversion).  This is made possible by the high clock speed.

1-bit Sigma-Delta (ΣΔ) uses two Prop pins, one for input, one for output.  You also need two resistors and two capacitors (for DC coupled inputs). 

Here's a typical setup:  

For more info, be sure to read Parallax's "Counters" Appnote.

Also, look at the Microphone2Vga demo .

This applet can help pick resistor values.

I've made an object and VGA demo that utilizes both counters of a cog to do simultaneous ΣΔ ADC of two inputs:

Screenshots:    

Download here.