What is a Signal Generator (Wikipedia link) - basically a signal generator is a device that can produce waveform's (like sine-wave, square-wave, triangular-wave, saw-tooth-wave's) at various frequencies. Commercial signal generators are fairly expensive- unless you are lucky enough to run across older ones for sale (I have not been that lucky yet - I keep looking though).
I have found a number of methods that may work using specific IC's designed as Signal Generators, Op Amps configured to produce different signals, function libraries for Arduino micro-controllers (audio signal generation), and DDS (Direct Digital Synthesizer).
I am currently working on all of these approaches and I will update my results with each after I get each one functional.
Resources (datasheets, etc.):
XR2206 Monolithic Function Generator IC
MAX038 Waveform Generator
ICL8038 Precision Waveform Generator
AD9850 (DDS IC)
(the first 3 are discontinued products but are still available on ebay)
UPDATES on the above Signal Generators