GLXOSD is an extensible on-screen display (OSD)/overlay for OpenGL applications running on Linux with X11 which aims to provide similar functionality to MSI Afterburner/RivaTuner OSD. It can show FPS, frame timings, temperatures and more in OpenGL games and applications. It can also be used to benchmark games, much like voglperf.
Eh, I guess this should be opened against the development branch.