Allegro
AllegAVI, Allegro, Allegro Webring, Allegro.cc, AllegroGL, AllegTTF, Allegutl, Gameprog, JGMOD, Striker
SDL
Adgali, Documentation Project, Drac Card Game Library, EZ2D, GFX with SDL, Kyra, libksd, Roger's SDL Tutorials, SDL Console, SDL Tutorial Wiki
3Impact
Impact, modelling and car library for games. Includes screenshots and downloads of demos. [Commercial]
Betatron
For the creation of 2D-platform and action-arcade games in C++ for the DJGPP v2.0 compiler. [Open Source]
ClanLib
A multi-platform game development library. Currently is works on Linux and Win32. [Open source, LGPL]
Clean Game Library
A complete development system for 2D games (arcade, side scrollers, platform games), using the functional language Clean.
DimensioneX Multiplayer Adventure Development Kit
Offers a freeware Java software capable of building and running multiplayer adventure games, playable with any browser. Several multiplayer games are available for free as a demo.
DRP Automotive Dynamics
Vehicles dynamics and damage modelling SDKs. Vehicle sample packs.
GAEM (Graphical Adventure Engine for Mobiles)
A game engine specially created to produce adventure games targeted to devices supporting MIDP 2.0 under Java microedition. [Open source, GPL]
GapiDraw
A complete toolkit to create games for handheld devices. [Free for non-comercial use]
Golden T Game Engine
A Java library for creating 2D games. [Freeware]
HGE: Haaf's Game Engine
A C/C++ library for creating hardware accelerated 2D games for Microsoft Windows. It provides minimal yet fully functional and flexible set of routines for handling graphics, sound and input. [Shareware]
IsoEngine
A Visual Basic/C++/.NET isometric/2d game engine. [Commercial]
Jet3D
A (commercial) free and open source realtime 3D graphics engine and SDK.
MIDAS Digital Audio System
MIDAS is a multichannel digital sound system. It supports DOS, Win32, and Linux platforms and plays Protracker (.mod), Scream Tracker 3 (.s3m), FastTracker 2 (.xm) and Impulse Tracker (.it) modules, as well as samples and audio streams in raw or WAVE . MIDAS is free for non-commercial usage.
MoleStudio SpriteCraft
An easy-to-use 2D game engine. It can be used in any programming language suppoting COM.
Object oriented Simple Directmedia Layer
OSDL, a free portable high-level open-source library for multimedia and game programming.
PathLib
Pathfinding library for games, optimal paths through maps, timing objects and their movements through maps. Sample application available.
RAD Gametools
Develops game software technologies which are licensed by primarily by video game companies.
RakNet
A game orientated networking API that is a wrapper for Berkely Sockets on Windows, Linux, and Unix. [Commercial]
The Grand Unified Toolkit (GUT)
A cross-platform C++ API for the development of games and other forms of interactive media. It is available for the BeOS, Linux/X, and Win32 platforms.
The Photon Effect
libCON DirectX C++ libraries and ColDet collision detection libraries. Documentation, tools, code examples, and game skeletons available.
The Yake Engine
A small, flexible, cross-platform game engine / application framework multi licensed under the LGPL and custom licenses.
Titan
A C++ based library to load/save game-related file formats, such as graphics, sound and 3d meshes.
VgaGames
A free library for programming vga games (320x200 pixels with 256 colors) for Linux and FreeBSD in C. [Open source, GPL]