What is your favorite tabletop game supplement or accessory?
I'm thinking of play advice books, setting books, splat books, GM advice books, and such; as well as dice rollers, initiative trackers, maps, card decks, tokens, and such. Anything that's not a core rule book or fundamental game component
(so, not counting dice in D&D or the player tokens in Monopoly). Includes card and board games as well as RPGs.
The image is by the ever-awesome +Daniel Solis
, based on observations by the ever-interesting +John Stavropoulos