Mesa provides a client-side (no GLX) pure software rasterizer.

Of course gnome-shell doesn’t work with it, so it should probably be blacklisted.

The glxinfo output looks like that:

name of display: :0.0
display: :0  screen: 0
direct rendering: Yes
server glx vendor string: Brian Paul
server glx version string: 1.4 Mesa 7.11
server glx extensions:
    GLX_MESA_copy_sub_buffer, GLX_MESA_pixmap_colormap, 
    GLX_MESA_release_buffers, GLX_ARB_get_proc_address, 
    GLX_EXT_texture_from_pixmap, GLX_EXT_visual_info, GLX_EXT_visual_rating, 
    GLX_SGIX_fbconfig, GLX_SGIX_pbuffer
client glx vendor string: Brian Paul
client glx version string: 1.4 Mesa 7.11
client glx extensions:
    GLX_MESA_copy_sub_buffer, GLX_MESA_pixmap_colormap, 
    GLX_MESA_release_buffers, GLX_ARB_get_proc_address, 
    GLX_EXT_texture_from_pixmap, GLX_EXT_visual_info, GLX_EXT_visual_rating, 
    GLX_SGIX_fbconfig, GLX_SGIX_pbuffer
GLX version: 1.4
GLX extensions:
    GLX_MESA_copy_sub_buffer, GLX_MESA_pixmap_colormap, 
    GLX_MESA_release_buffers, GLX_ARB_get_proc_address, 
    GLX_EXT_texture_from_pixmap, GLX_EXT_visual_info, GLX_EXT_visual_rating, 
    GLX_SGIX_fbconfig, GLX_SGIX_pbuffer
OpenGL vendor string: Brian Paul
OpenGL renderer string: Mesa X11
OpenGL version string: 2.1 Mesa 7.11
OpenGL shading language version string: 1.20
OpenGL extensions:
etc.