If you're interested in learning about the street trees in your neighborhood and you have an iPhone, check out the new Trees Near You app.
Each tree on the map is represented by a circle. When you click on a "tree" you can see the species and trunk size. The app also links to the wikipedia entry on the species in case you want to learn more.
For the moment, this app only covers New York City trees. This could be fun in Green-Wood... Thanks, Tibor.