This is the SparkFun MiniGen Signal Generator, a small shield for the Arduino Pro Mini that is capable of generating sine, square, or triangle waves at up to 3MHz, and approximately 1Vp-p. Though the MiniGen is technically a shield it can, in fact, operate as a stand-alone board capable of the the same functionality as it would if it were placed on top of a Pro Mini.
The MiniGen offers two out put sources for you to utilize: a 2-pin 0.1" spaced header and a u.FL connector. The output is at a DC offset of Vcc/2, however neither the offset nor the amplitude can be varied. By default, the MiniGen ships configured with a 3.3V regulator enabled and should only be used with 3.3V signals (or 3.3V Pro Minis if you are using this as a shield) unless the jumper on the back is soldered over to bypass the regulator for 5V use.