Pokemon Ruby/Sapphire

In-game reset:
Hold Select + Start + A + B during game play.
Information in this section was contributed by SkyDragon.

Delete saved game data:
Hold Select + Up + B. Then, answer "Yes" to the prompt to "Clear all game data areas?".
Information in this section was contributed by sfac.

Glitch: Changing glitch block (Japanese version):
Go to the fifth city and Surf on the route that has a sign on the water. Next to it is a glitched block. Swim back and forth and it will change.
Information in this section was contributed by Jason Howell.

Glitch: Battle without link cable:
Use a link cable, go into a Pokemon Center, and mix records. Come out and disconnect the link cable. Go to the person's Secret Hideout and they will be there. Talk to them, and you will battle them.
Information in this section was contributed by j.sherbaflanagan.

Glitch: Mix records in the Pokemon Center:
Disconnect the link and go to the Battle Tower and battle. You might see a person that has the same name and Pokemon as your "Mixing Record" buddy.
Information in this section was contributed by akmorrison.

Glitch: Jump sideways on the Acro bike:
Go into the building to the bike path. Get on your bike and go opposite from the door. Press B to do a wheelie and go back through the door. While the game is loading, hold Left or Right while still holding B. When you exit the door, you should jump left or right without turning.
Information in this section was contributed by kste7593.

Glitch: Cave Of Origin:
After Groudon is caught, go back to The Cave Of Origin. Go into the room with the magma pit. Walk around it, then have a Pokemon use strength. You will be in the pit of magma. Use Dig to get out.
Information in this section was contributed by ZacComicMan.

Glitch: Fall on a rock in Sky Pillar:
Note: You can only do this after defeating the Elite Four. When you enter the Sky Pillar, go to the second floor. Use the Mach bike and ride through the first set of loose floor (there should be eight), then through the next floors (there should be ten), and finally through seven more which should take you to the far left bottom of the second room. Next, go over one to the left and ride up to a small open space with two full tiles. You should be on the left one. Go over one and down one. You should fall and be on top of a rock. However, you can walk off without a problem.
Information in this section was contributed by NathanH1993.

Glitch: Shoal Cave: Tide level:
Go into Shoal Cave in the afternoon when there is no water (low tide). Save the game and turn off your Game Boy Advance; or keep playing but do not exit Shoal Cave. When you want to turn off your game, first save the in Shoal Cave at low tide and wait until morning. In the morning, it is high tide. Exit Shoal Cave then enter again. The tide is now high. As long as you stay in Shoal Cave, the tide will not change until you exit.
Information in this section was contributed by Kev Kiezun.

Glitch: Sky Pillar: Get to the top:
Have an Escape Rope. Get up Sky Pillar as far as possible. Use an Escape Rope and you will be at the top. Then, face Raquaza.
Information in this section was contributed by Scott Watson.

Glitch: Pokemon do not faint:
This trick requires a high level Pokemon that knows either Thief or Covet. It only works against a Pokemon that has the Sticky Hold ability. Find either a Grimer or Gulpin (Route 110 or Fiery Path) and attack with Thief or Covet. At a high enough level, you will instantly KO Grimer or Gulpin, and if they do not have the Sticky Hold ability, you will take any held item they have. If the Wild Grimer or Gulpin does have the Sticky Hold ability, its HP will hit zero, but a message will appear reading "Wild Grimer/Gulpin's Sticky Hold ineffective!" and the battle will end, although the Pokemon will not have fainted and fallen off the screen.
Information in this section was contributed by Ryutaiyo.

Glitch: Turning girl:
Outside of the berry master's house should be a girl. If you run in a circle around her, she will follow you by turning. Note: This is a glitch only for Running Shoes, not bikes, walking, or running.
Information in this section was contributed by purplekirby54.

Glitch: Repeating bike rider:
Go to Cycling Road. Keep going until you see the person going left and right. Go down to where you cannot see him and he will be going to the left. Wait until you see him going diagonally on the ground, then go down.
Information in this section was contributed by Austin Harris.