Character backgrounds! I first saw them used in 4E D&D, then later in Tales of the Grotesque and Dungeonesque (linked to the pdf, the backgrounds start on page 25). In both instances there is an in game benefit and a bit of an explanation of each. The TotGaD backgrounds have some interaction with the "leading question", an example being the alienist's leading question "The mad gibber unspeakable things; what was the strangest thing a patient ever said in your presence?" This can easily be used as a story/adventure hook.
Character do not need to to be spelled out, as seen in the first edition AD&D DMG, Gygax included a secondary skill table to indicate what your character may have been doing before taking up the adventurer's mantle. More recently, over at the Roll For Initiative web site, we have an article with something similar to Gary's secondary skill table except this expands it with separate tables for the different classes. The thought being what is a reasonable background for this class (ie. only a fighter will have access to the armorer background)
So, are there any generic fantasy backgrounds floating around? A cross between Gygax's secondary skill table and what's in TotGaD? If you have any ideas, let me know.