MS-20 Filter
The MS-20 is a synthesizer made by Korg and it has a pretty nice sounding filter. René Schmitz has made a popular clone of it and has schematics avavailable on his site....
Modules
Here’s a list of modules that have pages on the wiki.
Baby-8 CV Sequencer
The Baby-8 is a really simple and effective step sequencer built out of a CD4017(up to 10 steps) or CD4022(up to 8 steps) counter chip....
ADSR(-ish) envelope generator
This is a simple but effective envelope generator designed by Moritz Klein in his video. It approximates an ADSR by mixing together a short +12V peak and an attenuated version of the input signal and then feeding them into a capacitor through 2 diodes and potentiomters....