HarfBuzz is a text shaping engine. It primarily supports OpenType, but also Apple Advanced Typography. HarfBuzz is used in Android, Chrome, ChromeOS, Firefox, GNOME, GTK+, KDE, LibreOffice, OpenJDK, PlayStation, Qt, XeTeX, and other places.
Font Manager is an end-user oriented GUI to manage desktop fonts. It abstracts command line tools (fc-cache) away behind a pretty font preview browser and simple drag'n'drop installation method. It can also disable/enable fonts, collections, monitor directories, or add new sources. Recent versions are implemented in Vala and Gtk.
FreeType is a freely available software library to render fonts. It is written in C, designed to be small, efficient, highly customizable, and portable while capable of producing high-quality output (glyph images) of most vector and bitmap font formats.