Google Flight Search
Internally, this has been around for a little while (disclaimer: I work for Google but not on anything related to this).What continues to impress me about Google is:1. Just how quickly this was built (really, it was quick); and2. Google wants you to use our services because they're compelling not because we don't give you any other choice (ie "Don't be evil").Sure there are limits to what it currently does but I think you'll see it rapidly iterate.