Fontmatrix - *cool* A Linux font manager for both the home user and design professional. This software has individual user accounts, which allows each user to activate different fonts on a per-project basis.
Gentium - A Typeface for the Nations - A typeface family designed for diverse ethnic groups who use the Latin script to produce readable, high-quality publications. Support for a wide range of Latin-based alphabets; includes glyphs that correspond to all the Latin ranges of Unicode. Licensed under the SIL Open Font License, a Free copyleft license for fonts.
Linux.com Article - "A Font Primer for Linux" - An article about the different types of fonts (TrueType, OpenType, FreeType, Type 1, etc) and how these are incorporated on the Linux desktop. (2004) N.B.: The introductory overview is dated, but the details that follow are still quite useful today. (2009)
Opcion Font Viewer - A free and open source font viewer written in Java. It allows the user to view both installed and uninstalled fonts which makes selecting the right font quick and easy.
Unifont Homepage - *look* The best Unicode font site available; includes information about Unicode fonts, Unicode-enabled software, internationalization, and Unicode usability issues on free/libre/open source (FLOSS) operating systems.
Xft - the X Font Library - A client-side font API for X applications; uses Fontconfig to select fonts and the X protocol for rendering them.