How accurate (frequency and amplitude) do you need it to be? What frequency range? Does it have to be sinusoidal?
Here are a few options, listed in rough descending order in terms of "goodness" according to what I think you are trying to do.
0) Bust out the Cypress First Touch board from the 2009 KoP. The analog front end of that will easily do a current mode* controlled sine wave up to several megahertz. Of the DIY solutions, this is by far your best bet for cost and accuracy. The programming isn't too hard either, but expect to sink an afternoon or two into getting used to it - it is a funky combination of software and hardware.
1) Use the headphone jack on your computer - You'll need to calibrate the amplitude, but you can't get much cheaper.
2) Use a 555 timer - quick cheap simple, but it gives a square wave. You could filter it a bit...
3) Get a function generator kit, like this from sparkfun :
http://www.sparkfun.com/commerce/pro...oducts_id=9002
3b) Get a digital function generator kit and a microprocessor to control it:
http://www.sparkfun.com/commerce/pro...oducts_id=9169
4) Build up your own sine generator from a VCO (voltage controlled oscillator). This is solution is only good for higher frequencies - start here if you are above 40MHz.
5) Buy an actual function generator.
For most of these, you'll have to calibrate using an oscilloscope (you can use the cRIO's analog module as a good 'scope with a 500kS/s sample rate).
Again, I advise the cypress route. It has the best uncalibrated accuracy of the cheap options (3b and 5 are likely to be more accurate). Also, Cypress has expressed a lot of interest in helping FIRST folk with using their parts.
EDIT: * Current mode means that it is directly pushing current, not voltage. This means you don't have to worry about coil impedence. It also means that the internal DAC runs about 8 times faster than it does in voltage mode.