This is something OKCupid do in their emails. They have "Log in instantly" with a url that contains a unique hash. Even if you are not into dating, do check t

This is something OKCupid do in their emails. They have "Log in instantly" with a url that contains a unique hash. Even if you are not into dating, do check their site out... They actually have all sorts of nice stuff on their site - their labs section and blog are also very interesting if you like stats and data/analysis.