Benchmarks - 3DMark 11
3DMark 11 is the latest version of this hugely-popular synthetic benchmark. Making use of DX11 features such as tessellation, compute shaders and multi-threading, it provides modern-day results and is available as a free download.
Pushing a GeForce GTX 660 card close to its frequency limits still puts it behind a standard Radeon HD 7870 and the HD 7900-series-based HD 7870 Myst, though the gap is all-but gone at the highest resolution.