The Wien Bridge Oscillator
For a single frequency Wien Osc. you need 2 resistors (of the same value) and 2 caps (of the same value). To vary the frequency over a large range you require a dual gang pot for the resistors so that you can vary their values together. Then have say 4 frequency bands. Each band would be selected by switching in a pair of same value capacitors. 8 capacitors in all. This avoids the difficulty of trying to implement a dual gang capacitor.

Here's one I designed a while ago.