When I go to startup events I frequently encounter people who know me that I can't remember ever meeting. When that happens, if my wife is with me she will discreetly remind me, "She's the technical founder of such-and-such." You see, I'm terrible with both faces AND names.

Rapporative does the same thing, but in my inbox.

It's also great for figuring out someone's email address. Open up a new draft and start typing iterations (e.g., first.last@domain.com, first@domain, etc.). You'll know you've reached the right email when a profile pops up.