iOS Pick: JoyStickers
Megan Morrone's app cap on iOS Today 325 is the Joy of Tech sticker pack for Apple's Messages app. This is from Geek Culture, featuring the characters and creations of Nitrozac and Snaggy. and includes lots of entertaining animations and expressions.
In addition to posting cool stickers, you can place them on top of other messages. For instance, you could put a "Classified" message on top of a text, or you could put an animated propeller beanie on a photo.