Rovio's Angry Birds — one of the world's most popular mobile games — is now available for free from the app store. First noticed by Appsfire, the price drop from $0.99 (iPhone) and $2.99 (iPad HD version) represents the first time that the game has been available for free since its 2009 release. The free versions of both Angry Birds and Angry Birds HD— which were limited to a few playable levels — have been removed from the App Store in the US and UK. We've reached out to Rovio to see if this is a permanent move, or a limited-time offer.