The staff over at Instructables have designed and published their plans on how to make a nice Arduino-controlled waveform generator. Although you won't be replacing commercial equipment with it, the project is very easy to work with and they have also included plans for laser-cutting an enclosure to help move the project from the breadboard to a usable tool. You can generate sine, sawtooth, pulse and triangle waves of between 1 and around 50 kHz. Apart from the Arduino the rest of the circuitry fits nicely on an protoshield.
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