What's your favorite button on your toolbar?

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Currently I am using the Mozilla Firefox toolbar which is programmed with several useless buttons, including Firefox tutorials and news headlines. So, I have added my Gmail account and My Lot as buttons to the toolbar to make it more utilitarian. I'm thinking about adding other useful toolbar buttons, but can't think of any sites that would be worth visiting daily. What sites or buttons are currently on your toolbar? Which ones do you find most useful? Which sites would you recommend adding to a toolbar for daily use? Does Google offer cool toolbar options which might help? Would appreciate any suggestions. Thanks!