I've made a simple function generator to test out my homemade oscilloscope.  This single chip circuit (outside the power supply) is based on the Maxim MAX038, which I got as a free sample from www.maxim-ic.com.  It has tested OK up through the audio range (need to add an input amplifier to the o'scope to check out the higher freqs). 

Had a little trouble filtering the power supply...  Worked fine with battery power, but had to add many capacitors to work off of AC line power.  The 60Hz noise seemed to mess it up.

 The MAX038 is really a complete solution.  There are several other features that I'm not using such as voltage controlled frequency or pulse width modulation (see the pdf file).

 Pictures: 

 Circuit Diagram:      (click on thumbnail for larger image)

Photos of working box:      (click on thumbnails for larger image) 

 Next, I sawed short the shafts with a hacksaw and added knobs.  Then, I sealed up the case.  Seems to work well.