I noticed this on Hipmunk - I visited the site, and saw "Continue as {my name}." I clicked it and was signed in instantly. Wrote down on my task list to figure

I noticed this on Hipmunk - I visited the site, and saw "Continue as {my name}." I clicked it and was signed in instantly. Wrote down on my task list to figure out how Hipmunk was doing that, as it was a really neat user experience. It seems this is the answer (and they're even using Hipmunk in the demo screenshots). Would be shocked if this doesn't become very popular on websites.