IsoEngine - A Visual Basic/C++/.NET isometric/2d game engine. [Commercial] - http://soft.firstproductions.com/isoengine/
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/
The Yake Engine - A small, flexible, cross-platform game engine / application framework multi licensed under the LGPL and custom licenses. - http://www.yake.org/
Golden T Game Engine - A Java library for creating 2D games. [Freeware] - http://www.goldenstudios.or.id/products/GTGE/
3Impact - Impact, modelling and car library for games. Includes screenshots and downloads of demos. [Commercial] - http://www.3impact.com/
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/
Object oriented Simple Directmedia Layer - OSDL, a free portable high-level open-source library for multimedia and game programming. - http://osdl.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/
Clean Game Library - Full development system for 2D games (arcade, side scrollers, platform games), using the functional language Clean. - http://cleangl.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
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/
RAD Gametools - Develops game software technologies which are licensed by primarily by video game companies. - http://www.smacker.com/
GapiDraw - A complete toolkit to create games for handheld devices. [Free for non-comercial use] - http://www.gapidraw.com/
RakNet - A game orientated networking API that is a wrapper for Berkely Sockets on Windows, Linux, and Unix. [Commercial] - http://www.rakkarsoft.com/
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 . MI - http://www.s2.org/midas/
Jet3D - A (commercial) free and open source realtime 3D graphics engine and SDK. - http://jetpp.sourceforge.net/
ClanLib - A multi-platform game development library. Currently is works on Linux and Win32. [Open source, LGPL] - http://www.clanlib.org/