"Those will not only let you pay with a credit or debit card (and soon a special SFMTA card), but automatically adjust parking rates based on supply and demand, which means you could pay anywhere from $0.25 to $6.00 an hour depending on how many free spaces there are. Those rates are determined with the aid of some sensors that keep a constant watch on parking spaces, which also means you'll be able to check for free spaces in an area on your phone or your computer before you even leave the house."
See the parking meter website here.
Update 1 (22/8/2010)
Donald Shoup writes in favor of higher curb parking prices, which will deter crusining in search of parking space, which not only wastes resources and time, but also inconveniences other drivers and adds to the parking problems. See this, this and this.
Update 2 (3/6/2011)
The San Francisco Municipal Transportation Agency seeks to address the problem of traffic congestions with a twofold approach - to change the price of a parking space according to demand and thereby keep spaces open on every block, and to lead drivers to open spaces using an array of sensors, eliminating congestion caused by circling drivers.
The prices at 7,000 meters and 20 city-owned parking garages are adjusted with the aim of keeping two spaces available on every block. Drivers could pay from 25 cents to $6 an hour depending on demand. Currently, rates run from $2 to $3.50 an hour.