On Windows, there is Uniscribe, DirectWrite (since Windows 7) and also the legacy GDI font rendering APIs. Probably Microsoft will never remove any of these options to ensure backward compatibility.
On Mac OS X, you use CoreText - the legacy QuickDraw Text is deprecated and not even available for 64 bit applications.
There are also applications that do not use the platform font rendering, e.g. Adobe has their own font rendering code and current versions of Chromium and Gecko use Harfbuzz on all platforms.
On Windows, there is Uniscribe, DirectWrite (since Windows 7) and also the legacy GDI font rendering APIs. Probably Microsoft will never remove any of these options to ensure backward compatibility.
On Mac OS X, you use CoreText - the legacy QuickDraw Text is deprecated and not even available for 64 bit applications.
There are also applications that do not use the platform font rendering, e.g. Adobe has their own font rendering code and current versions of Chromium and Gecko use Harfbuzz on all platforms.