http://www.starmicronics.com/product/a.../index.cfm

Starmicronics makes a good buzzer. They have built-in tone genration circuitry and only require power on or off. An 8051 can switch its power through a simple transistor.

Make sure you select a 5 Vdc model, most are made for higher voltages.

You can control the enable timing to get either a quick "CHIRP" effect or the full listed tone.

There are many companies that make these devices. Look through their literature and three some keywords at yahoo to uncover more.

The buzzers are the simplest solution and I find them worth their low cost. And anything that requires only one 8051 pin is good in my opinion. :)

aka J