Newton Game Dynamics - open source from feb. 2011

Newton Game Dynamics is an integrated solution for real time simulation of physics environments.
The API provides scene management, collision detection, dynamic behavior and yet it is small, fast, stable and easy to use.

This API was created by Julio Jerez and Alain Suero.
Starting 02/15/2011 this API is open source and hoosted in google code.
See more here.

Allegro 5 library for game development.

Allegro 4 and Allegro 5 are cross-platform, open source, game programming libraries, primarily for C and C++ developers.

The Allegro library provides low level routines like input, graphics, midi, sound effects, and timing.
It is cross platform : Windows, OS X, Linux, DOS, and others.
The official website is here.
You can find some tutorials, and various articles for beginners.
Other information can be found here.