Sometimes a project calls for true AC waveform generation, whether it be generating an audio signal or making a DIY arbitrary signal generator. This Power DAC board made by Visigence for the Arduino Uno has 12 bit resolution, 15V rail to rail supply and can supply 250mA of current making it suitable for the majority of hobby projects. Sending voltage values to the board from an Arduino is done over SPI which means you can stack up to four of these boards on a single Arduino and easily control each one individually. Visigence provides schematics for the shield should you want to modify the design and some example code over on their github page.
One of the many interesting projects possible with this board is generating Lissajous figures on an oscilloscope. Visgence has made an interesting blog post about drawing Lissajous figures using this Power DAC shield. The basic idea behind these figures is you take two sine waves and feed them into an oscilloscope set to x-y mode. In x-y mode the scope will plot two input channels against one another. By varying the frequency and phase of each of the sine waves you can change the type of Lissajous figure that appears. Lissajous figures are better appreciated in video form. Check out this YouTube video which gives a great demonstration: