Basically, it's a filter, but instead of being a normal digital emulation of an analogue filter, it has two user-definable frequencies that are set apart from each other; as you move one over the other, it moves from being band-pass to band-reject (you can set one as high or low as possible for more normal effects). Not only that, but I seem to remember the slopes were infinitely steep, i.e. if you set LP cutoff to 500Hz, there's no sound at all above 500Hz - this allowed me to pick out the partials in a trumpet sample as I remember. It has a visual display, in which frequencies are displayed logarithmically from left to right and the spectrum of the sound migrates from the top to the bottom (like that thing in the Krishna synth, the name for it currently escapes me), so you can pick out partials visually and precisely. Obviously this allows you to create very weird sounds beyond the capabilities of a normal filter.
I would search, but have no place to start! A name would be very helpful if you know it, obviously the best would be if you can reference the magazine and issue as I should still have it. As far as I recall it was either a demo or freeware - I imagine the latter as I don't remember anything annoying about it.
Please help!

Edit: Oh, and if I find it myself, I'll be sure to post it here.