Get Geeks3D FurMark for Windows for free. This Windows-based stress test for graphics cards and GPUs is simple yet thorough. It's also a fast OpenGL benchmark.
For Windows, the Futuremark 3DMark Advanced / Professional Edition free download comes with all the tools you need to test both your computer and mobile devices. Depending on the device you're using to play games—a smartphone, tablet, laptop, or desktop PC—3DMark has a benchmark made just for it.