actually i have spend some thoughts about a little gadget for the last days.
I've found a little library for the atmel microprocessors that implements the USB stack via software. Creating a simple HID USB device seems quite easy with it, and since you can program the atmel any way you like, there are many possible ways of using it.
Currently i'm thinking about hooking up an old keyboard and adding 2 USB connectors to the atmel, which would result in a Y-connection for the keyboard.
And yes, i know that there are easier solutions for that, but it's a completly different thing if you can build/customize the tools yourself =)

I'm still missing some parts, but when i have them i will start playing around, and will post my progress on the forum.