Serenity USB Flash Drive
From ThinkGeek... store all your data with the best ship in the 'verse.
Engineered by Firefly Belt Buckle
Let them know your boat was made by the best. From Quantum Mechanix.
Serenity Playing Card T-Shirt
A cool way to show you're a browncoat while still sporting a colorful T-shirt.
Serenity Ornament
Could also look shiny dangling from your rearview mirror or whatever. Via ThinkGeek.
Serenity Plush
In case your bed just doesn't feel comfy enough without a Firefly-class spaceship among the pillows. Love the wee shuttles. From QuantumMechanix.
Malcolm Reynolds 1:6 scale figure
Possibly the most detailed Mal Reynolds figure ever, and they even pretty much got his face right. From Quantum Mechanix, but not shipping until 2015.
Firefly Clue Set
Figure out who helped the Alliance capture River. Spoiler alert: Jayne did it.
Days Of Not Taking You Seriously T-Shirt
Available via Entertainment Earth
Funko POP! Buffy the Vampire Slayer Figures
The Buffy figure is here, and there are also links to get Angel, Spike, Willow, a Gentleman, Oz, Goth Willow and a ton of others.
Titans Buffy the Vampire Slayer Vinyl Figures
These are supposed to be available by the end of the year from Titans, so I would just keep hitting refresh on this page.
Inevitable Betrayal Dinosaurs (With Sounds)
It's a bit older, but this ThinkGeek exclusive is still really, really nifty.