In the past I have built heartly oscillators to produce radio frequencies in the MHz range but I'm not sure what kind of circuit I would build if I wanted to produce much lower frequencies.

For example say I wanted to produce 30 KHz or even lower such as something like 30 Hz!

 

http://sound.westhost.com/project22.htm

 

A good old fashioned Wein Bridge would do it. It was the basic design used by HP for some of their first test equipment, a HP200.

Op amps are your friend.

http://en.wikipedia.org/wiki/Wein-bridge_oscillator

 

Here's a discussion about a few different oscillators:

http://www.calvin.edu/~pribeiro/courses/engr332/Handouts/oscillators.pdf

 

TI's "Op Amps for Everyone" is a very handy design guide for such things.
Link: http://focus.ti.com/lit/an/slod006b/slod006b.pdf
Look at chapter 15 for sinewave oscillators.