Using the library available from shaduzlabs, you can create arbitrary waveforms such as sine and square waves with an Arduino board. The library controls the PWM digital output and requires an external filter circuit for sine wave output. You can also create your own signals by entering the appropriate values into a lookup table for the library to interpreter.
The library is compatible with Arduino-compatible boards using the ATmega328, which includes our Freetronics KitTen, Eleven, USBDroid and EtherTen boards. For more information, circuit details and other related Arduino projects, visit their website. One of the benefits of using our Freetronics Eleven board is that you could build the filter circuit required for sine wave output right on the board, as the Eleven has an onboard prototyping area: