I've just been playing with Chua's circuit, using a couple of op amps for the negative resistance element.

It makes for some pretty XY plots on the oscilloscope. Thought you might like to see them.

 

 

 

It's very finicky getting the values right and you can see from the bottom trace that it's not very stable, but it's good fun to play with.

 

Here's a link to the follow-up blog where I go into more details about the circuit I was using: Op Amps: NIC (Negative Impedance Converter) and the Chua Oscillator