I'm looking for a portable, free library that supports user input, sound, and windowing system.
I'm using OpenGL for graphcis
Any suggestions?

You didn't mention what language you're using, but assuming C or C++ you could take a look at Allegro, SDL, or SFML. All three provide the features you mentioned, are cross-platform, and can be easily used with OpenGL.

I would recommend sfml. It is cross platform, has a clean and object orientated structure and is hardware accelerated by OpenGL. It also has bindings for other languages.

I would recommend sfml. It is cross platform, has a clean and object orientated structure and is hardware accelerated by OpenGL. It also has bindings for other languages.

The only thing you mentioned that is lacking from Allegro and SDL is the OO approach. So depending on what language the OP is using, that may or may not be a good thing. SFML also has networking. Allegro seems to have the more useful community support of the three, probably because it's been around for so much longer.

SDL is my personal favorite, but SFML is also a good library, especially if you are a strict C++ developer or a novice (though SDL is also good for novices).