Prepping for my "Unification Day" adventure for the Serenity Role-Playing Game at Nuke(m)Con 2008 sent me wandering the web for inspiration, references and shortcuts.
Unfortunately, the official Serenity web site at http://www.serenity-rpg.com is dead, but it seems Margaret Weis Productions is working on moving material from the old site to their new one. The most important of these is the PDF of the Official Serenity Character Sheet. The site also has news on the new Serenity Adventures book, which features a number of related scenarios for the game. The PDF is available now, the print version will hopefully be out soon.
The official site is down, but the unofficial one at FireflyRPG.com may be the next best thing. It's got a beautiful poster map of the Serenity solar system, 3D renderings of ground and air vehicles, weapon reference cards, and deck plans for 28 different ships. There are also two rules quick reference sheets, but unfortunately during the playtest I found they had too many house rules to be useful.
For only having 13 episodes and a movie, Firefly's managed to accumulate a heck of a lot of history. You can get an overview of all of it by checking out the "Firefly Timeline" at SerenityFirefly.com.
The Cortex RPG Web Site is a fan site for the core rule system that Margaret Weis Production used in Firefly and Battlestar Galactica. Fans with rules question about either game should check out the active forums on this site.
And finally, font geeks need to check out Adventures in Font Matching: Joss Whedon's Firefly, one man's quest to identify the font used to render the Firefly in its opening credits.