Phone Fun
In Pokemon Crystal many phone conversations have changed and sometimes people will even give you rare stones, so get as many phone numbers as possible to get a better chance of receiving a stone from one of your phone buddies.

Duplicate Items/Clone Pokemon
Go to a PC, and save. Attach the item you want to duplicate to the Pokemon you want to clone. Deposit it into the PC. Change boxes. When the screen is saying "Saving the game..." Turn off your GB. Turn it back on. You should have the deposited Pokemon in the PC, and a clone of it in your lineup. The item will either be on both monsters, or in your Backpack. Note: while doing this code, DO NOT withdraw any Pokemon. When the GB is turned back on, you'll find that the withdrawn Pokemon has been deleted.

Note I: You can duplicate up to 5 items/Pokemon at a time.
Note II: Use this code at your own risk. It may corrupt your save data if done incorrectly.

Pokemon Not in the Game
The following Pokemon cannot be found in Pokemon Crystal. You'll have to transfer them from Gold, Silver, Red, Yellow or Blue. As always, Mew and Celebi can only be gotten from special events or contests.

Mankey
Primeape
Vulpix
Ninetales
Mareep
Flaaffy
Ampharos
Girafarig
Remoraid
Octillery
Bulbasaur
Ivysaur
Venusaur
Charmander
Charmeleon
Charizard
Squirtle
Wartortle
Blastoise
Omanyte
Omastar
Articuno
Kabuto
Kabutops
Zapdos
Moltres
Mewtwo

Breeding/Evolving Pokemon
Breed two Pikachus to get Pichu.
Breed two Electabuzzes and you get Elekid.
Breed two Clefairies and you get Cleffa.
Breed two Jigglypuffs and you get Igglybuff.
Breed two Magmars and you get Magby.
Breed a Venonat with a Pinsir to get Pinsir.
Breed a Hitmonchan or Hitmonlee with a Ditto to get Tyrogue.
Breed a Clefairy with a Snubbull to get another Snubbul.
Breed a Gyarados with the Red Gyarados to get a golden Magikarp.

Mysterious Channel 13.5
Use your radio in the ruins of Alph to tune into Channel 13.5. You will hear mysterious sounds made by the Annon (Unown).

Suikun
Find Suikun (Suicune) in three different places (right out of National Park, below the bush by Mount Mortar, and up in Cianwood). Then, go to Tin Tower, and Suikun/Suicune will appear to fight you.

Serebii
Hook up the mobile adapter. Go to the communication center in Goldenrod City, and read the message on the lockers to the right of the room. They will talk about a present from the future. Leave and the blue-haired nurse will ask you to defeat all sixteen gyms in order to obtain the GS Ball. Once you have done so, give the GS Ball to Kurt. The next day, he will give it back. Put it in the shrine in the Ilex forest and a level 30 Serebii (Celebi) will attack you.

Annon
In the back of each puzzle room in the ruins of Alph is a word formed by Annons (Unowns). They require you to do things in order to get into a back room. In one place, you must equip a Water Stone. In another, you must put Houou (Ho-Oh) first in your lineup. In another you must use an Escape Rope. In another you must use Flash. Once in the back room, pick up the items and fall down the hole to find a room with a message about Annon/Unown. Fall down the hole there to return to the Annon/Unown shrine