The Irrlicht Engine is an open source high performance realtime 3D engine written and usable in C++.

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Developer: irrlicht.sourceforge.net
License: zlib/libpng License
Operating System: All 32-bit MS Windows (95/98/NT/2000/XP), All POSIX (Linux/BSD/UNIX-like OSes), WinCE, Linux, OS X, Solaris

K-3D is the free (as in freedom) 3d modeling, animation, and rendering system.

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Developer: k-3d
License: GNU General Public License (GPL)
Operating System: 32-bit MS Windows (95/98), All 32-bit MS Windows (95/98/NT/2000/XP), All BSD Platforms (FreeBSD/NetBSD/OpenBSD/Apple Mac OS X), All POSIX (Linux/BSD/UNIX-like OSes), Linux

D2X-XL is an OpenGL port of the classic 3D Shooter game Descent 2 for Win32, Linux and Mac OS X, containing many enhancements and bug fixes while preserving full backwards compatibility.

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Developer: descent2.de
License: GNU General Public License (GPL)
Operating System: 32-bit MS Windows (NT/2000/XP), Linux, OS X

Egoboo is a working cool 3D dungeon crawling game in the spirit of nethack.

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Developer: egoboo
License: GNU General Public License (GPL)
Operating System: 32-bit MS Windows (95/98), All 32-bit MS Windows (95/98/NT/2000/XP), All POSIX (Linux/BSD/UNIX-like OSes), Linux, OS X

A cross-platform, OpenGL-rendered Duke Nukem 3D port which provides many new features for mod authors.

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Developer: eduke32
License: GNU General Public License (GPL)
Operating System: 32-bit MS Windows (NT/2000/XP), All POSIX (Linux/BSD/UNIX-like OSes), OS Portable (Source code to work with many OS platforms)

Aleph One is an open-source descendant of Bungie's _Marathon 2_ first-person 3D shooting game.

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Developer: marathon
License: GNU General Public License (GPL)
Operating System: All 32-bit MS Windows (95/98/NT/2000/XP), All POSIX (Linux/BSD/UNIX-like OSes), OS X, Apple Mac OS Classic

Korax' Heritage is an ongoing project to create a new 3D action-RPG engine based on the original Hexen source code (DOOM engine), providing OpenGL/Direct 3D acceleration, new single- and multiplayer modes, lots of RPG features, new enemies, spells, etc.

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Developer: korax-heritage.com
License: GNU General Public License (GPL)
Operating System: 32-bit MS Windows (95/98), All 32-bit MS Windows (95/98/NT/2000/XP), All POSIX (Linux/BSD/UNIX-like OSes), Linux, MS-DOS

Hammer of Thyrion is a cross-platform port of Raven's Hexen II source.

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Developer: uhexen2
License: GNU General Public License (GPL)
Operating System: MinGW/MSYS (MS Windows), 64-bit MS Windows, All 32-bit MS Windows (95/98/NT/2000/XP), All BSD Platforms (FreeBSD/NetBSD/OpenBSD/Apple Mac OS X), All POSIX (Linux/BSD/UNIX-like OSes), QNX, FreeBSD, Lin

Real-time player and clan rankings and statistics for Half-Life.

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Developer: HLstats
License: GNU General Public License (GPL)
Operating System: All POSIX (Linux/BSD/UNIX-like OSes), OS Independent (Written in an interpreted language), Linux