We've featured a few ways to hide your data in plain sight, but DIYer Thice took a different approach: he actually hid a USB thumb drive inside his mouse.
Whether you're looking to hide a flash drive's worth of data or just free up a USB port on your computer, you can hide a USB dongle (whether it's a flash drive, a bluetooth module, or a Wi-Fi receiver) inside your mouse with a USB hub and a little ingenuity. You'll need to know how to solder, as well as (carefully) cut up the circuit board inside a USB hub, but it's a pretty neat hack if you have the room to fit all the parts in your mouse (or keyboard, or whatever else you might want to use). Hit the link to see how he did it.
Hiding Your Data in Plain Sight - USB Hardware Hiding [Thice.nl via Hack a Day]