I know a much more comprehensive list of these exists on the
GameDev.net forums. But for those of you who haven't seen that one or
just want a smaller list, here you go. I'll separate these by
language and broad feature sets.
C++
General
Gosu -
http://www.libgosu.org
minimalist 2D library that takes care of window creation,
input handling, loading files, rendering, playing audio, etc.
Torque -
http://www.garagegames.com
3D and 2D engines (we own a license to the 3D), handles
pretty much everything and now has some tutorials for adding a real
physics library
SDL -
http://www.libsdl.org
great little library that is open ended, does window
creation, input collection, etc. but leaves more up to the user than
Gosu does (also works well for building 3d on top of with OpenGL)
Graphics
Ogre3d -
http://www.ogre3d.org
fantastic rendering engine, full pipeline for exporting 3D
content and displaying with custom shader support
Physics
Open Dynamics Engine -
http://www.ode.org
mature rigid body 3D, known for stability, I haven't
played with this much but see it everywhere
Bullet -
http://www.bulletphysics.com
created by an ex Havoc employee, rapidly growing 3D
physics library with plenty of documentation
Box2D -
http://www.box2d.org
2D rigid body library, takes many ideas from Bullet.
Still not quite mature, but the core features are plenty stable and
development is still very active
Other languages (Ruby, Python, Lua)
General
Gosu -
http://www.libgosu.org
has Ruby bindings, what more needs to be said?
PyGame -
http://www.pygame.org
similar to Gosu, but for Python (not quite as object
oriented but still really cool). This is very mature and very well
known. It makes fewer assumptions than Gosu, and thereby leaves you
to do a little bit more work, but this just gives you more control.
Löve -
http://love2d.org
another one, but for Lua. This leaves the game loop up to
you, if you care to control the update loop.