Waveform generator is a handy tool in many situations. It allows testing other designs to see if they responds as you expected, measure characteristics of various components and do other stuff where various signal shapes are needed. Chris suggests using simple Arduino shield that makes it possible to convert it in to signal generator. Well waveform speed can be changed from 1Hz to 50kHz. This is practically enough for many basic tasks like debugging and testing.
Shield is equipped with three potentiometers that allow changing frequency, pulse width and gain. Four buttons with LEDs next to them allow selecting one of four waves including sine, triangle, pulse and saw. Signals are generated using cheapest method – R2R resistor ladder DAC. As Chris mentions, there are few drawbacks using this kit, but in other hand it is really simple to use and build.