What's the font in Mac OS X's GUI?
The font you see in most places on Mac OS X is Lucida Grande, it is the system font in OS X 10.1 Puma to 10.9 Mavericks. Mac OS 1-7 used Chicago, then Mac OS 8 and 9 used Charcoal.

The most recent additions are Helvetica Neue (OS X 10.10 Yosemite) and San Francisco (OS X 10.11 El Capitan through Mojave)