Toronto has this for streetcars and buses. It's run by NextBus, which seems to run GPS for a bunch of cities (http://www.nextbus.com/predictor/agencySelector.jsp).

They offer open access to this information through an API (see toronto.ca/open). I built something to watch the streetcars on a map using that API: http://totransit.ca/

My coworker and I were talking about letting people "check in" to specific streetcars, and tag variables such as "how full is the streetcar?", but I can't see many (any?) people actually doing that.