NVIDIA PerfHUD is a powerful real-time performance analysis tool for Direct3D applications.  PerfHUD is widely used by the world's best game developers (see screenshots and testimonials).

PerfHUD 5.1 Highlights:

• GeForce 8 Series support on Windows Vista and XP (older GPUs are also supported)
• Microsoft Windows Vista support (DirectX 9 and 10)
• New in 5.1! Instrumented driver updated to 163.71
• New in 5.1! Compatibility with Managed and XNA applications
• New in 5.1! Ability to tone map and save out textures and render targets
• New in 5.1! Complete changelist from PerfHUD 5.0 to 5.1 (.txt)
• Interactive usage model
• Shader edit-and-continue
• Render state edit-and-continue
• Revamped customizable user interface
• Automatic bottleneck identification
• Complete PerfHUD 5.1 feature list

http://developer.nvidia.com/object/nvperfhud_home.html#downloads