I clearly spent a little too long thinking about this, but it was fun nonetheless!
I’ve encountered a few issues on some sites that already capture keypresses, but for the most part it’s worked ok. I’ve only tested this on Chrome and Firefox (Mac, latest versions) but it should work fine on Windows. No idea about IE… Probably not, but I did try to use cross-browser methods where applicable!
The commented source is here and the script is enabled on the blog so you should be able to use it anywhere.
Oh, here are some sprites for good measure: