The Manaphy Code

At long last, Nintendo has revealed the code for obtaining the Manaphy Egg in Pokémon Ranger. You can find break-downs on both Pokémon Elite 2000 and Serebii.net. Here's the step by step instructions, as seen on Pokémon Elite 2000:

Step 1: Access Ranger Net after finishing the game. When you see Ranger net on your main menu, click on it.
Step 2: Your screen will show "Play a special mission." On your Nintendo DS, press the top Right trigger, the X button, and left on your directional pad simultaneously.
Step 3: You should see a screen that says "Enter the password has been added".
Step 4: Tap the touch screen, and wait as the game saves.
Step 5: After the game saves, tap the screen again, and a new menu will come up that says "Enter the password".
Step 6: When you click on the password menu, you'll be allowed to enter the password!
Step 7: Enter the password: P8M2-9D6F-43H7
Step 8: You'll see a special message that says the mission is active. Turn off the game.
Step 9: At the menu screen, access Ranger Net and you'll see an option to play a special mission.
Step 10: The mission "Recover the Precious Egg" appears!
Step 11: You did it! Congratulations! Now play the mission...
Step 12: ... and discover the mystery...
Step 13: ... of who Manaphy is.
-----
After receiving the Manaphy Egg in Pokemon Ranger, you will be able to transfer the Manaphy Egg to Pokemon Diamond or Pokemon Pearl so you can raise a Manaphy.
Info from Pokemon Elite 2000.

If you have Ranger, now would be the time to complete the game. Once that's done, you can get your Manaphy Egg and be one step closer to completing you Diamond and Pearl PokéDex. Good luck!