The 2-player trick
This one comes courtesy of Bruce (hangdog@ican.net), who posted this on Dave's MAME message board. In simultaneous 2-player games (Exed Exes was the example used) if you set the controls for both players the same, both of them will have the same movements as each other. Therefore, you can start a 2-player game, and both players will have the same movements, meaning that they can be combined for double the firepower! I have also found this to work quite well in Bubble Bobble.
Ms. Pacman Gets the Blues
There is a ROM bug in Ms. Pacman that will sometimes cause the maze to appear blue when you see it for the first time in the game. To see this for yourself, as soon as the "credits" screen in MAME disappears, quickly hold down the 1P button, and don't let go until the actual game appears. Sometimes when you do this, the first maze in Ms. Pacman will appear blue!
Green Beret Moonwalker
In Green Beret (or Rush n' Attack, as it was known in the US), if you start walking to the left then press and hold the right arrow key at the same time, your guy will start moonwalking! A thoroughly useless trick, as this often gets you stabbed by the soldier coming up behind you.
Level Warp in Crazy Kong
In Crazy Kong (the likely not all that legal clone of Donkey Kong,) on level 1 there is a way to warp right to level 2. After climbing up the first ladder, move barely to the right, and then jump too the right off the girder. What will happen is that you will fall through the floor and the level will end, just as if you had finished it normally! A good way to get a big bonus!
