Opening public data is a hot topic in Finland at the moment. As a small experiment with the data that is available I wrote buscatcher, a simple N900 app that displays Helsinki trams (and some buses) moving on a map in real time. This makes it easy to determine when your next tram is coming to the stop, or where it is stuck.
So far I’m keeping this application away from Extras until HSL gets scalability issues solved with their dataset. In the meanwhile you can grab and run the application from the GitHub repo.