The Alfa AS2164 quad vca chip is a great chip for making VCFs or VCAs. You get four independent voltage controlled amplifiers(VCAs) in a single package, so per-VCA cost is very low.
Each VCA has a 100dB dynamic range and accepts attenuation control voltages from 0V (0dB) to 3.3V (-100dB). Note that this is an exponential response of -33mV/dB. Negative control voltages can give gain up to +20dB. Supply voltage is ±4V to ±18V, so this is a chip that can also be used with stompbox 9V supplies.Download AS2164 datasheet
There is an example pole-mixing multimode filter circuit that can be built using this chip in the Multimode Filters, Part 2: Pole-mixing filters article, but the possibilities are endless!