How strong is spider silk?
Wikipedia has a remarkably well-cited article on the subject of silks and their various biological isoforms and mechanical properties. With respect to tensile strength, spider's silk is as tough as high-grade steel.

Explcitly, dragline silk was measured by Pérez-Rigueiro at al. to be 600 ± 50 MPa with a comparison to silkworm silk.

Reference:


 Pérez-Rigueiro J, Elices M, Llorca J, Viney C. 2001. Tensile properties ofArgiope trifasciata drag line silk obtained from the spider’s web. Journal of Applied Polymer Science 82: 2245–2251 [pdf]