The current revision of the specification is available here. Publicly available feeds are cataloged and archived at GTFS Data Exchange, and Google also maintains a list of public feeds. An analysis of archived public feeds in the summer of 2012 found that after a period of rapid growth, "about one fourth of the agencies in the U.S. have open route and schedule data, representing about 85 percent of the total passenger-miles served." Open transit data is also becoming widely available in Europe as EU member states move toward implementation of the PSI directive. Most of this data is provided in the GTFS format either directly by transport agencies or by third party data aggregators.
This server hosts a machine-readable catalog of feeds that helps avoid name clashes in areas where several agencies provide open data.