If you have a CRT monitor (the most usual ones people with desktop computers have), you can broadcast your voice or simply favourite music. How? With Tempest for Eliza. The program moves the cathodic ray tube [CRT] of you monitor, and the interferences fall in the AM band. Incredible. Read comments at Slashdot.
Update: you can also use your motherboad to broadcast in the FM band… read Slashdot’s story or visit author’s page (web page seems not to work, Google seems not to have it in cache).
Another update: this page is full of info about Tempest: http://www.eskimo.com/~joelm/tempest.html