Two really great apps are free this week. First up, Knights of Pen and Paper is an (in my opinion) underrated and very charming turn-based RPG, with the unique premise that you're actually playing an old pen-and-paper role-playing game, complete with a dungeonmaster describing every environment you enter and NPC you come across. Or you can play DM yourself, and design an adventure for your gamers. The game is on iPhone and iPad, so more users than ever have a chance to download and enjoy it.
And on the complete other end of the spectrum from underrated, the original Angry Birds app (and its HD counterpart) have gone free on iOS. It's hard to believe that you haven't played Angry Birds yet, as there have been lite versions, lots of sales and a number of various spinoffs and ways to play the game on all of its various platforms. But just in case you haven't gone back and played the original iOS hit, now's your chance.
Do try Knights of Pen and Paper, though. It's really good.