Arduino diy wireless
here there's a nice tutorial on how to obtain it.

Basically it sets the registers so that a pin outputs a PWM signal at 800 kHz and then modulates that signal in code by just switching the pin and off (pinMode set to INPUT or OUTPUT). If you plug a cable (20 cm and on) to pin 8, and tune your radio to 800Khz you'll hear some noise going on and off.

If you read down the forum you'll see people suggesting how to actually send tones, by putting a very small delay (delay(1)) between the on/off of the pin.

In this link they guys have ogn further and they even implemented a morse code library.

Now I am trying to figure out a way to do the same for a receiver.