In December 3DS users will be able to send messages to their other 3DS owning friends via a new app. Nintendo is calling this "Swapnote", which sounds like and resembles Flipnote Studio found on Nintendo DSiWare.  The app. will let you send 3D messages over both StreetPass and online.  It will let you hand-write messages, and also include pictures and recorded sounds.  It will be a free app. in the U.K. and will also be called Nintendo Letter Box. 

Source: Siliconera