HopStop is a service very similar to
Google Transit. It's a good alternative to use for cities that Google doesn't cover yet. HopStop has subway and bus directions for five cities: New York, Boston, Washington, San Francisco, and Chicago. These are all cities that are not yet on Google Transit. They list five cities as "coming soon": Philadelphia, Atlanta, Maryland, New Jersey, and Long Island. (Interesting that two of these are actually states not cities.)
The core of the service is just like Google Transit. Enter a starting point and ending point and HopStop will give you directions using public transit. Like Google, you can enter a starting day and time. It gives you the option to use the bus system, train system, or a combination of both.
In addition to directions, HopStop also has a city guide which is a search engine for nearby attractions, bars & clubs, hotels, restaurants, and shopping. HopStop also allows users to rate the bus and train routes. Users can get a sense of the quality of various bus and subway lines.
The final major feature is HopStop Mobile. Registered users can get directions via SMS or via voice by calling a toll-free number.
Overall, HopStop seems like a good service if you live in a city it covers. It'll be interesting as HopStop and Google Transit expand to include more cities and gain more users.