Those with Pokemon HeartGold and SoulSilver can download the legendary Mew for a limited time as part of anniversary celebrations.
As part of the tenth anniversary for both games, the Pokemon Company and Nintendo will offer you the chance to download #151 Mew using the Mystery Gift option when connected to the Nintendo Wi-Fi service. The adorable psychic critter comes with the move "Pound" and starts at Level 5. In his possession will be a neat Cherish Ball and Premier Ribbon.
To receive Mew head online between October 15 and 30th 2010 inclusive.