Here it is, by popular, frustrated demand: the master list of every secret (or not-so-secret) keystroke in macOS, including all the keys you can press during startup. Clip and post to your monitor (unless, of course, you got this book from the library).
For the most part, the following list doesn’t include the keystrokes already listed in your menus, like -P for Print, -S for Save, and so on.
Starts up from a CD, DVD, or USB thumb drive
Starts up from a network server
Puts the Mac into FireWire or Thunderbolt Target Disk mode
Starts up from a startup volume
Starts up to diagnostic or hardware test mode
Starts up to diagnostic or hardware test mode over the Internet
Starts Recovery Mode
Starts up from Recovery Mode over the Internet
Shows icons of all startup disks and partitions, so you can choose one for starting up
Starts up from external drive (or CD)
Zaps the parameter RAM (PRAM) ...