Swap Note

Swapnote, or Itsuno ma ni Koukan Nikki in Japan (Nintendo Letterbox in the EU, Nintendo Mailbox in some other areas) is a free messaging app on the Nintendo 3DS released in late December of 2011. Unlike other messaging apps that let you give small blurbs of text, you essentially send scribbled notepad messages which are "redrawn" to the recipients, and even have the ability to draw in 3D along with attaching 3D pictures or sound bites. These messages can be sent to friends online via SpotPass or person-to-person StreetPass.

Aside from the ability to scribble notes, it is possible to make Yonkoma, RPGs, and if skillful enough, small animations. It is even possible to set up Alternate Reality Games given the ability to pass messages from person-to-person. You can also unlock various Stationery and features to use that can be purchase with the Nintendo 3DS's in-system currency Play Coins.

Perhaps most interesting of all is the popularity it has garnered not just from its messaging functions but from Swapnote's guide in the form of a Mii called Nikki (in the bottom left side of the pic to the right). Already various fanart of her have popped-up since the app debuted. At New Year's Day at 2012, Nintendo themselves sent out a new stationery (that can be acquired by simply having a friend send a message with the new stationary to you), along with a QR Code of Nikki's own official Mii usable in Mii Maker.

Has nothing to do with a certain black notebook where having a person's name written on it shall die. Although it can be that...

Tropes used in Swap Note include:

Exposition Fairy: Nikki.
Meaningful Name: Nikki is Japanese for journal/diary.　Koukan nikki is a form of exchange jounal among close friends.
Meganekko: Nikki.
Role-Playing Game: This video provides an example of how to create a simple tabletop RPG within Swapnote.
Spiritual Successor: To the DS's Pictochat.
Stop Helping Mii: Nikki can sometimes be this. Luckily you can skip though her messages stating you have unlocked things like attachments and 3D drawing. Which leads to...
Unlockable Content: The ability to send pictures and sound attachments along with 3D writting are unlocked as you save/send more messages. Additional Stationery can be purchased via Play Coins.
Yonkoma: You are given up to four pages for each message you send.
Variable Mix: The background music changes when reading, writing, or browsing your notes.
Video Game Cruelty Potential: You can send anything to anyone in your friends list. Even that.