I had just read an article on Fender tremolos here. This article brought up the subject of the Fender three tube Vibrato/Tremolo. This is a tremolo effect that pans between two signals, one from a High pass and the other from a low pass. The sound while not truly tremolo and not really vibrato has a unique flavor that has qualities of both.
I’ve seen exactly two projects that make use of this. The first was a project from the now defunct Stompboxology Newsletter named Tremolo-Matic X. The other is the Vibrotrem by Bajaman ove at Freestompboxes.org. Basically the circuits do the same thing though they use different methods. The input signal is split and routed through a high pass and low pass network. A tremolo effect is applied to each of these signals with the cycles being 180 degrees out of phase. As the high pass increases in volume the low pass signal is decreasing and vice versa.
The Tremolo-Matic X uses a NE570 chip as two VCA. It uses a couple op-amps for a LFO and a few more as a buffer and the high and low pass networks. I built this and posted some notes here and here. The circuit had a great sound and was not difficult to build. The NE570 may be hard to get or cost prohibitive to some.
The Vibrotrem uses a LED/LDR to control the level of the high and low pass sections. Bajaman seems to have done thorough study of this type of tremolo and has optimized the filter section. I have not built this project, but have no doubt it would sound as good or better.
Caitlinbread makes a box, the Pareidolia that does a similar effect, without the controls for Bass and Treble. From their description it sounds like the Bass and treble fix at an optimal point where they share a midrange cross over. The clips on the site sound pretty good.