If Dr. Google is your primary care physician, and you're more likely to see Dr. Pepper than some guy with a stethoscope, fear not:
The SF Free Clinic lives up to its name based on availability, provided you have no insurance of any kind. They do wellness check-ups for kids and adults, disease screening, reproductive services, etc. on a pay-what-you-can basis. Appointments are an absolute must.
San Francisco Free Clinic
4900 California St
San Francisco, CA 94118