Xvfb is, sadly, largely unmaintained.

The new hotness is Xdummy - any distribution with XOrg 1.12+ and xf86-video-dummy 0.3.5+ (Ubuntu 14.04+, Debian 7+) can spawn a headless X server which, unlike Xvfb, has full access to extensions. This means Xdummy can be used to run OpenGL tests, whilst Xvfb cannot. 

directhex@flame:~$ Xvfb :900 -fbdir /tmp/foobar
directhex@flame:~$ xpra --xvfb="Xorg -noreset -nolisten tcp +extension GLX -config /etc/xpra/xorg.conf +extension RANDR +extension RENDER -logfile ${HOME}/.xpra/Xorg-10.log" start :100
directhex@flame:~$ DISPLAY=:100 glxinfo | grep ^direct
direct rendering: Yes
directhex@flame:~$ DISPLAY=:900 glxinfo | grep ^direct
libGL error: No matching fbConfigs or visuals found
libGL error: failed to load driver: swrast
Error: couldn't find RGB GLX visual or fbconfig