Gmail Mouse Hack
At Sudama's office today, I plugged a Kensington Optical Mouse into my powerbook and remapped the buttons to be gmail actions. Right click is "archive" and the bottom left and right clicks use the vi/gmail mappings for up and down. Clicking the left and right buttons simultaneously launches gmail in your default browser. Now that I have mapped mouse buttons to vi navigation keys, I feel like the cycle of UI design is complete once and for all.
Mouse and other I/O applications usually offer application level customization. In the brave new world of thin clients and web-centric applications, users will want to customize these buttons at a web-server or even page level.