HarfBuzz
HarfBuzz is an OpenType text shaping engine. There are two HarfBuzz code trees in existence today:
The current HarfBuzz tree, also known as harfbuzz-ng, is under active development and is what is used in Firefox 4. It is available here.
The old HarfBuzz tree, derived from FreeType, Pango, and Qt, is available here. It is in maintenance mode and not actively developed. We expect users of this code base to start moving to harfbuzz-ng soon.
Background
To get a better idea of where HarfBuzz stands in the text rendering stack you may want to read State of Text Rendering. For an intro to the ongoing API design issues with the new HarfBuzz see this thread, although note that the thread is from August 2009, and all of the issues raised there have been resolved already.
Mailinglist
HarfBuzz discussion happens on harfbuzz at lists freedesktop org.
IRC
HarfBuzz people frequent the #harfbuzz channel on freenode.
Bugs and Patches
Please report bugs (and submit patches) through the Bugzilla.
Download
There are no tarball releases of the old HarfBuzz codebase available. If you want to use the old HarfBuzz, just import it into your project.
For tarball releases of the new HarfBuzz codebase, look here. The API is not expected to change incompatibly, but we cannot guarantee that until HarfBuzz 1.0.0 is released.
For developmental snapshots, look here.