This is probably the biggest spider I've ever seen in San Diego. Any ideas?

I'm gonna say Neoscona crucifera, but I can't say for certain. It's definitely a harmless orbweaver though; anything that makes a big traditional spiral web like that is going to be harmless. That's not really an hourglass anyway, more of a liberty bell shape.

In fact, in San Diego the only medically significant spider is the western black widow. They make messy cobwebs and you'll know when you see one.

The picture makes him look pretty small, but his abdomen was about the size of a large grape. We think it's an Orb Weaver... but it has an hourglass on it's belly.