Pokémon Yellow - Special Pikachu Edition

Cheats

Powerful Pikachu at the Start
To get a powerful Pikachu at the start, trade it over to one of your other games and raise it up just like normal up to level 100.  Once you trade it back, it will obey you just the same.  The only catch is that it will not learn any of the new moves that Pikachu learns in this game, unless you have a lot of patience to train against those level 2 Pidgeys.

Get Raichu to Follow You
Trade your Pikachu over to another game and evolve it.  Once you trade it back, there is a chance that you will have a Raichu following you.  Be careful when you do this, though.  Once you get the Raichu, it's impossible to get back unless you have a copy of Pikachu stored somewhere.

Duplicate Pokémon in a Link Trade
To duplicate Pokémon in a link trade, trade over the Pokémon you want copied and turn off your Game Boy before saving and have your friend save his game.  Once you turn on your Game Boy, you will have your old Pokémon and so will your friend.  However, you will lose a Pokémon in the process, so go out and catch something like a Caterpie or other low leveled Pokémon in abundance.