I have a buddy who swears by his Evoluent Vertical Mouse. It’s basically a regular mouse flipped on its side so that instead of having your hand parallel to the floor when you mouse, you’ve got it in a handshake position. It comes in models for both right and left handers.
Is a vertical mouse more ergonomic than a regular one? According to the Evoluent manufacturer, their mouse eliminates one of the culprits leading to RSI: forearms twisted into an unnatural position. Having never tried a vertical mouse, I can’t share my personal experience, but here are a few reviews from around the web incase you’re thinking of buying one:
Evoluent VerticalMouse 2 Review
Evoluent VertialMouse Review
Evoluent VerticalMouse 2 promotes ergonomic “handshake” positioning
The Evoluent 2 is available from Amazon for just under $60: Evoluent Vertical Mouse 2