GTFS Data Exchange
GTFS Data Exchange is the largest global GTFS feed registry for schedule transit data, and is used by many 3rd party application developers to discover GTFS data for various cities around the world. Agencies can register their GTFS feed URL (e.g., http://my.agency.org/google/google_transit.zip) with GTFS Data Exchange, and GTFS Data Exchange automatically regularly retrieves and archives a copy of the data if it has changed. As a result, many agencies' GTFS data is archived going back years on the site.
If you are a transit agency and would like your name and data to appear on GTFS Data Exchange, see the instructions to be added here.