Beginning in October 2010, Muni began to switch over from using paper Fast Pass to an RFID-based smart card called Clipper Card. As of May 2011 all Fast Passes were phased out.
If you live and work in San Francisco, you may want to consider buying a Fast Pass A or M. For $72 per month, Fast Pass A allows you to ride Muni and BART for an unlimited number of times within the city! For $62 per month, Fast Pass M allows you to ride Muni for an unlimited number of times within the city. If you only ride the bus to and from work five days a week, you would otherwise be spending $80 per month!
Fast Passes are available at Muni stations about 5 days before to 5 days after the beginning of each month, and at several supermarkets and convenience stores. Once you own one, you have a grace period of three days after the end of the month to continue using it on MUNI (BART does not provide the same grace period). A May Fast Pass will work until June 3rd, for example.