Diamond Dust

On the following days, Fly up to Mt. Silver and instead of your usual snowfall, there will be glittery diamond snow:

February 29th (leap year day)
March 15th (Ides of March / Honen Matsuri)
October 10th (release of Pokemon Blue in Japan)
December 31st (New Year's Eve)

Contributed by: Eevee-Trainer 

Unlock 8 bit music

Once you obtain all 16 badges, talk to the music designer in Celadon, and you unlock the 8-bit originals from the original gold/silver games.

Contributed by: big_one12 

Level Up Unhatched Pokemon

To do this glitch, you must first execute the Pomeg Berry Glitch, which is done by using a Pomeg Berry on a Pokemon with 1 or 2 HP so that its HP loops to 65,535 (displayed as ?35 in-game). Do so, however, with only one Pokemon and an Egg in the party, with the Egg in the top-leftmost slot of the party in the Pokemon menu. After executing the glitch, you will now be able to enter battles and use the Pokemon Egg as if it were a normal Pokemon. It will function as a normal Pokemon - you can view the stats it will have upon hatching, it can level-up by beating Pokemon, it can learn new moves, it can even evolve. This means that you can alter the Pokemon inside the Egg in otherwise-illegitimate ways. For example. Pikachu cannot normally know Thunder when hatching from an Egg at Level 5; through this glitch, Pikachu can. Similarly, you can hatch Eggs with fully-evolved Pokemon such as Typhlosion, Meganium, and so on, inside.

Contributed by: KeyBlade999 

Pomeg Berry Glitch

For this glitch to work properly, you must have a Pomeg Berry and a Pokemon who has not had their HP lowered as much as possible already through Pomeg Berries. (If the latter is the case, the use of HP Ups on the Pokemon will suffice.) Get the Pokemon to a severely low level of HP (1 or 2 HP). Once you do so, in the field, use the Pomeg Berry on the Pokemon. This will in turn cause the data in their HP to cycle back to the top of the counter, giving the affected Pokemon 65,535 HP - over 90 times the otherwise possible maximum in the game.

Contributed by: KeyBlade999 

Unlockable Pokewalker Maps

There are over 20 Pokewalker maps currently available to anyone and everyone who owns them. The trick is obtaining Watts and getting the National Dex. Note that some of the following are event-only, so the Yellow Forest (for example), is unattainable right now.

Contributed by: marioman1154 

Pokedex Diplomas

Meet the conditions below, then speak with the Game Freak director in Celadon City to get the following diplomas.

Note that Mew, Celebi, Jirachi, Deoxys, Shaymin, Darkrai, Manaphy, Phione, and Arceus do not count towards this, and are thus not needed.

Contributed by: KeyBlade999 

National Dex

Contributed by: raytan7585 

Trainer Card Stars

By completing certain tasks in the game, you get stars added to your Trainer Card. (Of special note are the Shiny Leaves, found only by speaking with your Pokemon in certain areas on very rare occasions, the locations being Nature-dependent.)

Contributed by: KeyBlade999 

Diamond Dust

On the following days, Fly up to Mt. Silver and instead of your usual snowfall, there will be glittery diamond snow:

February 29th (leap year day)
March 15th (Ides of March / Honen Matsuri)
October 10th (release of Pokemon Blue in Japan)
December 31st (New Year's Eve)

Contributed by: Anonymous 

Level Up Unhatched Pokemon

To do this glitch, you must first execute the Pomeg Berry Glitch, which is done by using a Pomeg Berry on a Pokemon with 1 or 2 HP so that its HP loops to 65,535 (displayed as ?35 in-game). Do so, however, with only one Pokemon and an Egg in the party, with the Egg in the top-leftmost slot of the party in the Pokemon menu. After executing the glitch, you will now be able to enter battles and use the Pokemon Egg as if it were a normal Pokemon. It will function as a normal Pokemon - you can view the stats it will have upon hatching, it can level-up by beating Pokemon, it can learn new moves, it can even evolve. This means that you can alter the Pokemon inside the Egg in otherwise-illegitimate ways. For example. Pikachu cannot normally know Thunder when hatching from an Egg at Level 5; through this glitch, Pikachu can. Similarly, you can hatch Eggs with fully-evolved Pokemon such as Typhlosion, Meganium, and so on, inside.

Contributed by: Anonymous 

Pomeg Berry Glitch

For this glitch to work properly, you must have a Pomeg Berry and a Pokemon who has not had their HP lowered as much as possible already through Pomeg Berries. (If the latter is the case, the use of HP Ups on the Pokemon will suffice.) Get the Pokemon to a severely low level of HP (1 or 2 HP). Once you do so, in the field, use the Pomeg Berry on the Pokemon. This will in turn cause the data in their HP to cycle back to the top of the counter, giving the affected Pokemon 65,535 HP - over 90 times the otherwise possible maximum in the game.

Contributed by: Anonymous 

Unlock 8 bit music

Once you obtain all 16 badges, talk to the music designer in Celadon, and you unlock the 8-bit originals from the original gold/silver games.

Contributed by: big_one12 

Unlockable Pokewalker Routes

With regular usage of the Pokewalker included with SoulSilver, you can obtain Watts that can be used to unlock special courses that hold unobtainable pokemon in both HeartGold and SoulSilver versions. Watts are not total obtained, but collected in the game after returning from a stroll or sending gifts back to your game. There are also several routes unlocked by Nintendo Wifi Events and specific pokemon.

Contributed by: PersianGato 

Pokedex Diplomas

Meet the conditions below, then speak with the Game Freak director in Celadon City to get the following diplomas.

Note that Mew, Celebi, Jirachi, Deoxys, Shaymin, Darkrai, Manaphy, Phione, and Arceus do not count towards this, and are thus not needed.

Contributed by: Anonymous 

National Dex

Contributed by: raytan7585 

Trainer Card Stars

By completing certain tasks in the game, you get stars added to your Trainer Card. (Of special note are the Shiny Leaves, found only by speaking with your Pokemon in certain areas on very rare occasions, the locations being Nature-dependent.)

Contributed by: Anonymous