Hopstopa site that helps you map your travel plans in major cities in the USisn't perfect. As many readers of TechCrunch point out, it doesn't always point you to the most efficient routes, and it sometimes overestimates the time it will take you to get places. But it's still the next best thing to local knowledge when you're traveling on public transportation in San Francisco, Boston, New York, DC, or Chicago. And as someone who can get disoriented in the mall, I love the 360 degree photo they provide that tells me which way to walk when I get out of the last station.
In San Francisco, you can also take advantage of a neat service provided by NextBus that uses GPS tracking devices to tell you when the train or bus will be arriving at your station. Now that HopStop has released its free API, perhaps someone will mash up the two services so that I can plan my journeys downtown to the second.