Started by Goodman Games, Free RPG Day is simple: visit your FLGS (friendly local game store, but you knew that already) and pick up free stuff. Simple as that. It's a great way to encourage visits to that vanishing breed, the game store. And hey, while you're at it, support them. You need not buy anything to get a free goody or two, but why not support your local store while you're there?
I shop online as much s the next guy, RPG stuff included. But I sorely miss my vanished game stores. There's something special about visiting a brick & mortar store and actually flipping through that new game supplement or hefting that latest Fantasy Flight Games (free plug) board game monstrosity, not to mention the open gaming (which a surprising amount of stores still offer).
So visit your local store(s) and grab some free booty! Hey, maybe I'll see you there.