I've been happily using the nvidia drivers for a while now, but now I need to test something in Mesa (I need to find out if a problem I'm having on another machine is related to Mesa or a piece of hardware). I reinstalled the the Mesa packages (I'm using Debian) so my libGL.so file in /usr/lib points to the Mesa library, and I changed the "nvidia" driver to the "nv" driver in my xorg.conf file. The problem is whenever I run anything like glxinfo it says "Xlib: extension "GLX" missing on display ":0.0"." There's a GLX library in /usr/lib/xorg/modules/extensions/lib, but that seems to belong to nvidia (X complains about being unable to find an nvidia X driver when it tries to load that GLX library). Renaming that glx library to something else just makes X complain that "(EE) Failed to load module "glx" (module does not exist, 0)". Any ideas?

Why exactly did you change your driver fro nvidia to nv?

Huh, I guess I thought I had to disable the nvidia driver to get Mesa to work. I switched back to the nvidia driver and Mesa worked just fine. Thanks!