I usually only call for car accidents I've seen. I actually saw one happen live, and I received a call a couple of hours later from a detective who wanted my description.
Besides that, I called 911 for a couple of homeless crackheads I saw slumped over in front of a vacant storefront. They sent a fire truck, two cop cars, and an ambulance (slow morning?). Naturally, the crackheads got up right when the fire truck got there and the police handled the rest.