OpenAL is, like the name implies, and Open Audio Library, developed by Loki Games in cooperation with the soundcard maker Creative Labs.
The idea of the library/API is to create a cross-platform audio output library that is capable of multi-channel output and environmental audio. It also supports voodoo stuff like out-of-the-box doppler effects.
The name is a bit like OpenGL due to fact that most OpenAL calls are similiar to OpenGL - so, when working on the 3D graphics, it's probably possible to make the 3D sound at the same time. In a way, just like OpenGL it's meant to compete with Microsoft's DirectX.
Home page: http://www.openal.org/