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 - Full 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.
GapiDraw - A complete toolkit to create games for handheld devices. [Free for non-commercial use]
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.
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]
3Impact - Impact, modelling and car library for games. Includes screenshots and downloads of demos. [Commercial]
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.