Synths
These are not intended to be exhaustive, nor are they replacements for the manuals. These are intended to make it easy to remember hidden features and how to access them. They are also based on my own usage, so for many synths, the sequencer sections are ignored, since I don't use the built-in sequencers on many synths.
Some useful hidden functions aren't documented if I don't yet understand how to use them. For example, the modulation options in the Korg Minilogue XD are completely opaque to me.