Generation 1 GlitchdeX

Missingno. is a glitch in Red, Blue, and Green. It appears rather late in the game, and can really both help and harm your game.
How to access:
1. Speak to Old Man in Viridian City, and have him show you how to catch Pokemon.
2. Fly to Cinnabar Island.
3. Surf up and down the coast repeatedly. You will encounter Missingno, 'M, and other seriously glitched Pokemon.
4. After you meet the glitch himself, the sixth item in your bag will be multiplied to 128. Also, your Hall of Fame data will be glitched up.

WARNING!!
A small number of cartridges can't handle Missingno. Try at your own risk, as some game data may be deleted on said cartridges.

Okay, um, nice topic. I meant to make a spot for pokedexes. It might be moved there after a while.

Dokokashira Door Glitch- try at your own peril!

Exclusive to the Japanese Red & Green

1. Obtain Oak's Parcel.

2. Enter the Pokémon list window. Press Select next to your Pokémon, and then hit the B button.

3. Enter the Pack subscreen. Highlight Oak's Parcel, and then press Select.

4. The game will now believe that you are fighting using Oak's Parcel, and will run into all kinds of glitches. The most notable (and fun to exploit) is that warp points suddenly become psuedo-random. By taking a certain number of steps and entering your rival's house, you can actually bypass the entire game, and enter the Hall of Fame in less than 20 minutes.

5. Beware! Teleports are seemingly random. If you black out after a battle, you will be warped back home. Your Pokémon will be poisoned with 1 HP left, meaning that if you try to go inside to heal your Pokémon, you will be caught in an infinite blackout loop. NEVER SAVE IF THIS HAPPENS!!!