The world’s most successful iOS app celebrated its second birthday yesterday (11 December, 2011).

To mark the occasion, developer Rovio released a new “Happy Birdday” update, which features 15 new birthday cake levels as well as updated menus, graphics and special achievements. Also, all episodes are now unlocked to all players. You can download the update from the iTunes store now.

Angry Birds has been downloaded 500 million times, making it the most downloaded iOS app released to date. It is also considered one of the fastest growing international brands.

Various retailers around the world celebrated the occasion with special promotional deals, including Barnes & Noble, Toys ‘R’ Us, Forbidden Planet and even the Apple Store in Ginza, Japan.

Remember, you won’t find Angry Birds in the South African iTunes store. Check out our guide to creating a US iTunes account if you haven’t already.