I just built one that is pretty successful, using an LM324, a couple of diodes, two yellow LEDs, and some resistors. Oh, and a cap.
It amplifies the ac fluctuations of the forward voltage of one diode, while keeping the dc component the same, and compares it against the other diode. The second LED is for keeping it lit all the time. I wrapped both LEDs in a piece of yellow paper. It flickers quite nicely, and is affected by airflow. It actually looks quite a bit like a candle if you unfocus your eyes... ;)
One could probably use the Vf of the two LEDs instead of using diodes...
Regards, Bob Monsen