SF has lots and lots of parking meters. Maybe not enough, or maybe too many (depending on your take on things). There's still lots of free parking throughout the city.
Types of meters
SF has a few different flavors of electronic parking meters, most installed during 2008-2014.
Colored parking meters
Green parking meters can be used for short-term parking.
Yellow and red parking meters are for commercial vehicles only (with commercial license plate) during the posted hours. Outside of the posted hours they can be used for regular car parking, provided you follow the rules of the curb color. Read the sign.
Transition to electronic meters
In July 2008, the city of San Francisco started an 18-month pilot program to replace at least 6,000 traditional parking meters with centralized electronic pay stations. The project was intended to study how people's driving habits will be affected by longer payment hours, variety of payment methods and changes in parking fees, with the long-term goal of relieving traffic and the hassle of finding parking, through text message and email notification of available spaces. An unintended result of these parking meters' removal is the possible loss of thousands of parking spots for bicycles. More information about the program can be found here.
The electronic meters also feature solar panels on their back side.
All meters are supposed to have been upgraded to the electronic type as of 2014. Prior to that, meters were only coin-operated.
The battle over sunday meters seems complicated. Leaving this section as available for anyone who wants to write more!