Mesa is a collection of free and open-source libraries that implement several rendering as well as video acceleration APIs related to hardware-accelerated 3D rendering, 3D computer graphics and GPGPU, the most prominent being OpenGL. Mesa is hosted at freedesktop.org and used on Linux, BSD and other operating systems. Additionally to the APIs, Mesa also harbors most of the available free and open-source graphics device drivers, which is a bit misleading, since the actual device drivers are located in the kernel, and the user-space part consists of some compiler software. The development of Mesa started in August 1993 by Brian Paul, who is still active in the project today, by now containing numerous contributions from various other people and companies worldwide, due to its broad adoption. Crowdfunding has been successfully used to partially drive development of Mesa.
Mesa is known as housing implementation of graphic APIs. Historically the main API that Mesa has implemented is OpenGL, along with other Khronos Group related specifications (like OpenVG, OpenGL ES or recently EGL). But Mesa can implement other APIs and indeed it did with Glide.