It's probably on the main page's entry for Subterranea, but here goes. Fight your way to the third Hexuplex and wipe out all the Aerobots. When the crystal appears, shoot it a number of times (either 33 or 43 I think) and it will change to the initials MK for Mark Klein. If you shoot it again, it'll change back, keep shooting it a LOT and the initials will re appear then disappear again on the next shot. I can't remember right off, but I vaguely recall reading that Subterranea's difficulty will reset at a certain point.
Dragonstomper: Find the flashing castle and unlock it, then defeat the two maniacs inside to receive a Paper. This item will let you cross the bridge without paying 600 in gold. (It actually serves as ID that the guard is asking for).
Tunnel Runner: Some of the doors have a weird symbol beside them. If you face one your score will increase but this only works the first time you look at it. The symbols are actually the letters db on one side and cb on the other, for the programmer Dick Balaska and his wife Cindy. Also rotating the joystick in a full circle will teleport you to a random location, but this only works once per maze. If you lose a life, it'll work again. I honestly thought this was a bug when I first found it. Oh, if you're missing the white insert, read it for some updates to the game.
Yars Revenge: I can confirm through gameplay that the HSWWSH is triggered by the Ghost of Yar, NOT by repeatedly tagging the Swirl.
No Escape: Again, I'm not sure if this is a bug, but notice how the Furies move when you throw a stone. Use this to guide them into falling bricks. Also, after so many waves, the difficulty resets.
Demon Attack: I have a picture label and a text label version. One ends at wave 84 and the other doesn't.
Robot Tank/Space Shuttle: We discussed it a while back and found that one or the other (or both) has two different bankswitching schemes. One version works on a 7800 while the other doesn't.
River Raid II won't run on a six switch console.