We've seen so many fake Mew codes pop up on the web (and we get about 20 new fake Mew code submissions A DAY) that we've decided to add this following disclaimer to our codes section:

According to Nintendo, Mew cannot be found or caught in Pokemon Yellow, Red, or Blue. Outside of a GameShark code (which may mess up your Pokedex and render the game incompatible with Pokemon Stadium), the #151st Pokemon remains a cool litte extra that is only handed out by Nintendo on special occasions, such as Mall Tours or Pokemon events.

However, all those who want to catch a glimpse of the rare Pokemon can select Mew as a rental Pokemon in Pokemon Stadium for Nintendo 64 (but you cannot upload it to your Game Boy game). Note that Mew is just that -- a bonus Pokemon (albeit a very cool one that can learn all TMs) -- and not technically required to complete your Pokedex.