PBuffer Rendering
Basically, FBconfigs and PBuffers allow you to do off-screen rendering
with OpenGL.  The OSMesa interface does basically the same thing, but
fbconfigs and pbuffers are supported by more vendors.
PBuffer rendering may also be hardware accelerated.
PBuffers are getting more use nowadays, though they've actually been
around for a long time on IRIX systems and other workstations.
The
http://oss.sgi.com/projects/ogl-sample/registry/SGIX/fbconfig.txtGL_SGIX_fbconfig and
http://oss.sgi.com/projects/ogl-sample/registry/SGIX/pbuffer.txtGL_SGIX_pbuffer
 extensions describe the functionality.
More recently, these extensions have been promoted to ARB extensions (on
Windows at least).
The Mesa/progs/xdemos/ directory has some useful code for working
with pbuffers:
pbinfo.c - like glxinfo, it prints a list of available
    fbconfigs and whether each supports pbuffers.
pbutil.c - a few utility functions for dealing with
    fbconfigs and pbuffers.
pbdemo.c - a demonstration of off-screen rendering with pbuffers.
Mesa 4.1 and later support GL_SGIX_fbconfig and GL_SGIX_pbuffer (software
rendering only).
