3D Engines Database: Provides a list of 3D engines and their features, along with reviews, ratings, and screenshots of each engine

Allegro: A free and popular game programming library

Crystal Space: A free (LGPL) and portable 3D engine written in C++. It runs on GNU/Linux, Windows, and MacOS/X and supports many modern 3D features like shaders, volumetric fog, stencil shadows, physics, skeletal animation, 3D sound, ..

Simple DirectMedia Layer: A cross-platform multimedia library designed to provide fast access to the graphics framebuffer and audio device