Continuing our new The Little Things series that highlights the often-overlooked polish and underrated features that make Mac OS X such a joy to use, I thought I'd highlight one of those 'guy behind the guy' features that makes Apple's OS so gorgeous: font anti-aliasing. Nerdy, I know, but check out the screenshot: Windows, even XP, doesn't support this feature system-wide like Mac OS X does, and it shows. Type looks like garbage in everything from desktop icons to most applications and their menus on Windows. Mac OS X, on the other hand, supports anti-aliased fonts from the ground up (to my knowledge), so everything from System Preferences to desktop icons, text editors to iLife and more are incredibly legible and lickable.
Some call it a minor detail, but given the undeniably pleasant usability this brings to the OS, I would argue it's one of those trademark additions that Apple's engineers don't receive enough credit for.
Update: As many people pointed out Windows does, in fact, have a similar feature called 'ClearType,' which some consider superior to OS X's (though it is a matter of taste). The key difference is that ClearType is disabled by default, which in effect means that most Windows users have no idea that it is even an option. Another case of Apple paying attention to the little details, though Vista will have this feature enabled by default.