There is actually a fairly good list here: https://support.google.com/websearch/answer/2466433?hl=en https://support.google.com/websearch/answer/2466433?hl=en

There is actually a fairly good list here: https://support.google.com/websearch/answer/2466433?hl=en https://support.google.com/websearch/answer/2466433?hl=en I know for a fact that it isn't complete. But most of the "secret" ones that I am aware of were very obscure and usually buggy, so maybe this is all of the officially supported ones.