Historically the state of OpenGL debugging and profiling tool is quite a mess unfortunately, especially on OSX and Linux. D3D always came with a matching PIX for capture-analysis (capture frames and step through them), and NVIDIA's PerfHUD for live-analysis (analyse the running game with a realtime overlay). For OpenGL there was gDebugger which was commercial, then free, and hasn't been updated in years and probably is stuck to an old GL version. Then there's NVIDIA Nsight which seems to be limited to NVIDIA GPUs (but has a very nice VisualStudio integration). Apple only seems to care about GLES on iOS, not desktop GL. So VOGL could really fill a gap.