If used with software that could keep up, a scroll wheel mapped to send a stream of 'undo' and 'redo' events could be kind of cool.

"Borrow Your Laptop", by Randall Munroe, licensed under Creative Commons Attribution-NonCommercial 2.5 License

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If used with software that could keep up, a scroll wheel mapped to send a stream of 'undo' and 'redo' events could be kind of cool.