My first tree of 2009 is the Coast Live Oak (Quercus agrifolia). Of all the trees in Golden Gate Park, this is the only species that is native there....and there's a whole lot of them. It's fun to learn a tree you see a lot, because everytime you walk by one you can say "coast live oak" (say it out loud and give it a wave).
How do you know if you're looking at a Coast Live Oak? Their height is pretty variable, from 25 to 40 feet, so height could throw you off. To be sure, check out their characteristic leaves.
Coast Live Oaks have round to oval shaped, brittle leaves, generally with several spine-tipped teeth. Look for tufts of hairs on the bottom of the leaves.