Several libraries are built on top of or beside OpenGL to provide features not available in OpenGL itself. Libraries such as GLU can be found with most OpenGL implementations, and others such as GLUT and SDL have grown over time and provide rudimentary cross-platform windowing and mouse functionality, and if unavailable can easily be downloaded and added to a development environment. Simple graphical user interface functionality can be found in libraries like GLUI or FLTK. Still other libraries like GLAux (OpenGL Auxiliary Library) are deprecated and have been superseded by functionality commonly available in more popular libraries, but code using them still exists, particularly in simple tutorials. Other libraries have been created to provide OpenGL application developers a simple means of managing OpenGL extensions and versioning. Examples of these libraries include GLEW (the OpenGL Extension Wrangler Library) and GLEE (the OpenGL Easy Extension Library).
In addition to the aforementioned simple libraries, other higher-level object-oriented scene graph retained mode libraries exist such as PLIB, OpenSG, OpenSceneGraph, and OpenGL Performer. These are available as cross-platform free/open source or proprietary programming interfaces written on top of OpenGL and systems libraries to enable the creation of real-time visual simulation applications. Other solutions support parallel OpenGL programs for Virtual Reality, scalability or graphics clusters usage, either transparently like Chromium or through a programming interface like Equalizer.
Mesa 3D is a free/open source implementation of OpenGL. It supports pure software rendering as well as providing hardware acceleration for several 3D graphics cards under Linux. As of 22 June 2007 it implements the 2.1 standard, and provides some of its own extensions for some platforms.