GPU-Z is a simple but reliable application that displays a lot of important information about your video card and graphics processor. It supports all the GPUs from all the popular manufacturers of such devices, including Intel, AMD, ATI, and Nvidia.
GPU-Z impressed me mostly by the neat way that it structures all the vital information which it displays, using only its main window. You can easily see information like the DirectX support, the pixel fillrate, the bus width or the driver version. A lot of adapter and GPU-related information is displayed, including clocks data (both overclocking and 3D clock parameters will be shown if available). A separate window will display real-time sensors info, including the GPU temperature, the fan speed, the GPU load or the memory used.
Anyway, GPU-Z won't only display GPU-related data. It can also validate the results, perform a GPU load test to verify PCI-Express lane configuration, or back up your graphics card BIOS.
Considering that it's also free, GPU-Z gets my vote of confidence, as it's a really nice tool for retrieving GPU data and monitoring its real-time parameters.