What kind of tree should I plant?

Considering that most trees have the potential to outlive the people who plant them, the impact of this decision is one that can influence a lifetime. Match the tree to the site, and both lives will benefit.

First decide what tree features you want:
Trees are Good has a good discussion of how to identify your needs.

Now use this search tool to help you identify the exact tree that meets your needs:
Urban Forest Ecosystems Institute has a database of 1481 trees with 49 attributes. This is the grand-daddy of tree search engines; it's from Cal Poly San Luis Obispo.

For a second but smaller opinion, also check:
City of Los Angeles, Urban Forestry Division, Tree Selection Guide gives you a search of a 150-tree database useful in Los Angeles and of course San Diego.

Next, if this is going to be a street tree, i.e., in your parking strip, check to see if the City of San Diego shows your choice as an "acceptable" street tree:
The San Diego, Street Division, Urban Forestry has tree information specific to San Diego.  Click on Tree Selection Guide to find the street trees acceptable for City of San Diego residents. If you don't like any of these, you may request approval from the Street Tree Division.  Regardless of the street tree you wish to plant, all street trees planted in City of San Diego need to be approved via a No Fee Street Tree Planting Permit.

If you already have a tree and want to know what kind it is:
Look at the Arbor Day Foundation, What Tree is That? to use an automated tree ID program which asks you questions and narrows down the identity of your tree. This does not have all trees growing in the west, but it contains many common ones.

And finally, some free stuff:
The Arbor Day Foundation gives away 10 free trees with levels of membership (some would even grow in our climate zone).