Libraries

Betatron
For the creation of 2D-platform and action-arcade games in C++ for the DJGPP v2.0 compiler. [Open Source]
http://www.geocities.com/SiliconValley/Horizon/8630/

ClanLib
A multi-platform game development library. Currently is works on Linux and Win32. [Open source, LGPL]
http://www.clanlib.org/

Clean Game Library
Full development system for 2D games (arcade, side scrollers, platform games), using the functional language Clean.
http://cleangl.sourceforge.net/

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.
http://www.dimensionex.net/en/

GapiDraw
A complete toolkit to create games for handheld devices. [Free for non-commercial use]
http://www.gapidraw.com/

Golden T Game Engine
A Java library for creating 2D games. [Freeware]
http://www.goldenstudios.or.id/products/GTGE/

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.
http://gut.berlios.de/

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]
http://hge.relishgames.com/

3Impact
Impact, modelling and car library for games. Includes screenshots and downloads of demos. [Commercial]
http://www.3impact.com/

IsoEngine
A Visual Basic/C++/.NET isometric/2d game engine. [Commercial]
http://soft.firstproductions.com/isoengine/

Jet3D
A (commercial) free and open source realtime 3D graphics engine and SDK.
http://jetpp.sourceforge.net/

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.
http://www.s2.org/midas/

Object oriented Simple Directmedia Layer
OSDL, a free portable high-level open-source library for multimedia and game programming.
http://osdl.sourceforge.net

PathLib
Pathfinding library for games, optimal paths through maps, timing objects and their movements through maps. Sample application available.
http://pathlib.hildebrand.cz/pathlib.html

RAD Gametools
Develops game software technologies which are licensed by primarily by video game companies.
http://www.smacker.com/

RakNet
A game orientated networking API that is a wrapper for Berkely Sockets on Windows, Linux, and Unix. [Commercial]
http://www.rakkarsoft.com/

Regimental Command
A generic armored combat system.
http://regcom.sourceforge.net/

Titan
A C++ based library to load/save game-related file formats, such as graphics, sound and 3d meshes.
http://titan.sourceforge.net/

VgaGames
A free library for programming vga games (320x200 pixels with 256 colors) for Linux and FreeBSD in C. [Open source, GPL]
http://www.vgagames.de/index_en.html

The Yake Engine
A small, flexible, cross-platform game engine / application framework multi licensed under the LGPL and custom licenses.
http://www.yake.org/