I was in the process of creating a wave exhibit for G.Wiz, and I realized that a simple, portable variable square wave generator would be a useful thing to have. Unfortunately, I was limited to RadioShack parts (sorry, RadioShack, but five IC's in the whole store ain't gonna cut it), and therefore couldn't use any chips intended for this purpose. After fruitlessly trying the old trick with a potentiometer  connected to the control pin of the 555 and realizing that doing so would impact the duty cycle as well, I realized that what I really needed was a way to change the capacitance with a potentiometer, and got this circuit.

The center of the page is what you should pay attention to (the stuff on the left is just to be able to modulate an exterior power supply, plus power supply caps and diodes and all that stuff you don't need me to tell you about). Note that instead of tying the capacitor straight to ground, I instead tied it to a potentiometer, with the center pin attached to another capacitor which is tied to ground, and the opposing pin also tied to ground. What this did is essentially allow me to selectively put the second cap partially or fully in series with the first, which will decrease the total capacitance. If the center pin is swung all the way to the ground terminal, however, both sides of the secondary cap will essentially be tied to ground, removing it from the circuit and allowing the full wavelength that the primary cap would otherwise provide. Because you are only changing the capacitance, the duty cycle will be unchanged, granting true independent duty cycle and frequency control.

NOTE: this is a placeholder. The full instructable, with photos and a board layout, will come as soon as I have time to finish it.