Hidden gems or tricks in OS X Yosemite (10.10) In addition to this question I'd like to start a new question specifically for the new OS X Yosemite. 

As I'm always curious about the announced items, there are many times hidden gems or nice tricks in new OS X versions.

Please share your hidden tricks (one per answer)! Please do not add "default" tricks which are available in all previous versions.
Using currency calculator directly in Spotlight by typing in a number and a currency. Spotlight is automatically showing the most used conversions.