AD9833 Waveform Generator
Julian Ilett has a look at a AD9833 Waveform Generator.  It looks like a nice device. Connect a microcontroller to it and use SPI to interface to it. You can set it up for square wave, sign wave or triangle wave all through SPI.  The AD9833 looks like it could be very useful whenever you need to easily generate specific frequencies. The module that Julian is using was under $7 with free