These aren't really Brownies, but the pun was too good to pass up. They are fay, however, members of the Seelie Court, whose enemies are the Unseelie Court (Spriggans always seem to show up). This has been a very popular game at conventions, but the genre isn't written up anywhere - nor am I willing to explain it to you, any more than is written here. Too much work.Basically the PCs are none-too-bright members of King Oberon's court who are sent to the Mundane World on some mission or other. They are only about eight inches tall (~20 cm), can become visible or invisible to humans as they wish (though are always visible to animals), can talk with animals, are bound to keep any promises they make, like to reward kind humans and play tricks on mean humans, and so on. Great fun!
This genre especially appeals to women for some reason - maybe it's the love potion that Oberon gives them to reward a kind human or play tricks with on mean humans ...
And, yes, I'm aware that Puck and Robin are two names for the same character in most legends - but not in this game ...