How many games have you played that had cheats? Until recent year, almost every game had some secret way you could gain instant money, skip levels and so forth. Super Mario Bros had a secret passageway on course 1-2; the original Legend of Zelda allowed you to skip half the game if you named your character "Zelda".
My games are no exception. Cheats are helpful in aiding testing, like adding a secret button for skipping levels or unlocking every item to see if they work like they're supposed to. However, I have never revealed any of these secrets... until now.
Super Invader - starting with purple box
You didn't expect Super Invader to have cheat, did you? It has. To activate it, shoot the little alien on the main menu screen and then quickly click the appearing boxes first in the upper-left corner then the one in the lower-right before they disappear. Once activated, every game starts with a purple box spawning for both players. Not very useful, but it might be fun for a few rounds.
Counter Invader - Instant millionaire
Money cheats are one of the simplest and most common cheats available in video games since they're both fun for the player and useful for testing. I added one into CI in order to test every shop option for mix ups (like pressing 5 for a SMG and getting a shotgun). To activate it, play until you get into a shop screen and simply type "change" into the game. Kat-$hing! Instant 100,000. As an afterthought this is kind of silly, because Counter-Invader is taking place in a quasi-utopian communist state.
WW2 - Bush Battle
In the original release of Bush Battle, my first proper game for Windows, you'd think the only cheat is simply moving in a straight line where there's no shooting going on. Well, that's kind of cheating, but you could also shoot a secret tree in the far right corner on level 6 to... ugh, repeat any bloody level you have already completed. Fairly pointless unless you really, really, really want to beat that top score. (PROTIP: You don't.)
The rest - Macintosh games
Before I made Bush Battle for Windows with Game Maker 5, I was using HyperCard on the Macintosh to make a variety of games. These games are unavailable for download because they require HyperCard and MacOS Classic to run. Also, they're stuck on the old Performa. However, when the day comes that I manage to finally port them to Windows you'll be able to skip through chapters by using the following cheats. For what it's worth.
New Quest - Leads to the intro (Oh wow, pointless). Ghost Busters - Puts you right after meeting the first ghost. Key Item - Puts you 4 rooms later after finding the key. The Cellar - Down the cellar right after. Dots on Ground - 2 rooms later. Mystical Trail - 2 rooms later again. What's This? - First useful cheat since it takes you past the death traps. The Rush - Near the Light Orb. The End - Final Battle of wits.
Once you've played through Dark Shade the first time there's also Dark Shade extreme. In this mode you can find various easter eggs and there's a couple of twists. That's about it for Dark Shade, though.
PATCHEDGLUM - leads to Chapter 1 (Again, pointless). BROTHERHOOD - Chapter 2. ENFORCING - Chapter 3. SLAUGHTER - Chapter 4.
Moon Light is full of weird easter eggs and secrets. Just playing around with it I discovered I could find cheat notes under sand spills. There was this weird random room with a picture of a man in a hat and a boom box (in 1926?) and all kinds of pointless things. Moon Light's actually fairly fun to play despite being a 7 years old game. Dark Shade in comparision is rather crude.
Final game! I was kind of expecting a cheat in this game but all I could find was some weird clue at the final screen about setting my screen resolution to "12-inch screen" whatever that means. It promised easter eggs but I couldn't figure it out.
Anyway, that's it. Apart from Slime Slayer X, there's the cheats for almost every game I have released in full. Though for a final note, here's the cheat for Slime Slayer X. This is it, just press H at the first screen. This will unleash the massive powers of Ha-do-ken and obliterate the poor slime into smudge. Poor poor slime, what did it ever do to you? Think about it.