In anticipation of this one i’ve been looking at my collection and generally trying to think of what actually qualifies as an RPG accessory. Off the top of my head I can only think of couple of things that would genuinely count as an accessory and not a sourcebook.
The first is the DM Screen. This is a quintessential RPG product that dates back to at least 1st ed AD&D and possibly longer. I own a great many screens, for a variety of different games, some good, some bad. A screen isn’t the must have item that it used to be, some games don’t have official screens and others don’t even require the DM to pick up a dice, let alone hide their rolls, but they are still an important tool for the DM, providing easy access to tables and lists that save watching time searching through rulebooks. I’m a big fan of the DM screen.
After that, what else is there? Well there are player handouts, but they tend to come as part of an adventure than a stand alone accessory. Call of Cthulhu has some of the best examples, with Beyond the Mountains of Madness having it’s own accessory pack of handouts and with every single pre-written adventure coming with pages of letters and clues for you to photocopy for use in game. Call of Cthulhu also has packs of forms with such things as birth and death certificates and Sanatarium Admission forms, all designed to add a little depth to your game. Few games make as good a use of the handout as Call of Cthulhu but, used right, is another great tool that came really help make a game.
Lastly, at least from my own collection and off the top of my head, there are audio accessories. I covered these in a little in my article on atmosphere in gaming, but here bear mentioning again. Of all the various accessories these are probably my favourites because, used right, they can transform a game. Planescapes Mimir in the Planers primer to the Outlands is one outstanding example, as is the Ravenloft adventure A Light in the Belfry. Others might not like them as much as I do but I think that they can change the tone of a game if they are employed correctly.
Also falling into this category are the excellent sounds effects and soundboard tools produced by Battlebards. To me anything that draws the players into the world further and helps them suspend disbelief is a good thing and adding an audio element to the game is simple and effective way to do that.