As of this post, there two canon ASOIAF additions:
Currently 3 in this series: The Hedge Knight, The Sworn Sword, & The Mystery Knight. They follow the adventures Ser Duncan The Tall and Aegon Targaryen V. The time period is just under a 100 years prior to the events of Game of Thrones, 10 years or so after the Blackfyre Rebellion.
A stand-alone short story written in a more omniscient style: historically documented instead of POV. It chronicles the civil war known as the Dance of the Dragons. The time period is 200 years prior to the events of Game of Thrones.
There are also two known upcoming/planned additions:
- Another Dunk & Egg tale ("The She-Wolves of Winterfell")
This was delayed in favor of publishing The Princess & The Queen, and is expected to be released as part of a collection of the first 3 Dunk & Egg tales.
A coffee-table illustrated book, not a story. It will touch on many events throughout the ASOIAF history, with lots of new trivia expected to be revealed. Its release has been delayed many times, but is currently set for release in October 2014.