...FreeType2 for external font support (+ HarfBuzz for Unicode text shaping) mpg123 for better MP3 audio support. WildMIDI for better MIDI audio support. Libvorbis / Tremor for Ogg Vorbis audio support. opusfile for Opus audio support. libsndfile for better WAVE audio support. libxmp for better tracker music support. SpeexDSP for proper audio resampling.
A modern, cross-platform Quake game engine based on FitzQuake.
QuakeSpasm is a *Nix friendly Quake Engine based on the SDL port of the popular FitzQuake. It includes some new features, important fixes, and aims for portability and 64 bit correctness.
Hammer of Thyrion (uHexen2) is a cross-platform port of Raven Software's Hexen II source. It is based on an older linux port, Anvil of Thyrion. HoT includes countless bug fixes, improved music, sound and video modes, opengl improvements, support for many operating systems and architectures, and documentation among many others.