Crystal Space is a free (LGPL) and portable 3D Game Development Kit written in C++. It supports: true six degrees of freedom, colored lighting, lightmapped and stencil based lighting, shader support (CG, vertex programs, fragment programs, ...), mipmapping, portals, mirrors, alpha transparency, reflective surfaces, 3D sprites (frame based or with skeletal animation using cal3d animation library), procedural textures, particle systems, halos, volumetric fog, scripting (using Python, Perl, Java, or potentially other languages), 16-bit and 32-bit display support, OpenGL, and software renderer, font support (also with freetype), hierarchical transformations, physics plugin based on ODE, ... See the extensive list of features for more details.
Crystal Space currently runs on GNU/Linux, general Unix, Windows, Windows NT, and MacOS/X. It can optionally use OpenGL (on all platforms), SDL (all SDL platforms), X11 (Unix or GNU/Linux) and SVGALIB (GNU/Linux). It can also optionally use assembler routines using NASM and MMX.