Following my Firefox tip, here is another one that can save your life if your are a clueless European in the US - works the other way around too.

Google has a nice calculator feature which knows (oohooh magic!) how to convert most currencies and more importantly measure units. That really covers the essentials:

What is "20 miles" in kilometers?
Is 75F cold? (for a beer or for a bath...)
Are 5 gallons that much?
Is that girl on mySpace really that sexy? (I know, that's bad...)
How expensive is that super-super-size-me value meal?