There are a wide variety of resources dedicated to the production, maintenance, and analysis of GTFS. Below are a number of useful resources, grouped by type.
GTFS.org - Website that contains detailed information on the components of static and real-time GTFS, as well as best practices and additional resources
Google Transit - Contains technical documentation on static and real-time GTFS, as well as instructions on how to prepare and submit feeds to Google Maps
GTFS-Realtime - Technical documentation of the GTFS-Realtime spec from Google Transit
MobilityData GTFS Validator - Command-line Java tool for validating static GTFS feeds
National RTAP - GTFS Builder - Free-to-use spreadsheet-based GTFS feed creation tool, intended for use by small/rural/tribal agencies.
World Bank - Introduction to the General Transit Feed Specification (GTFS) - Online course on GTFS feeds and their uses.
GTFS2HTML - Open-source code to turn GTFS feeds into easily-updateable HTML timetables
Hundreds of applications can ingest GTFS data. Submit your GTFS feed to major trip planning apps, the National Transit Map, and GTFS feed aggregators through the emails in the list below:
National Transit Map: nationaltransitmap@dot.gov
Google Maps: transit-partners@google.com
Apple Maps: maps-transit@apple.com
Transit App: data@transit.app
Bing Maps: mapstransit@microsoft.com
Moovit App: usa@moovit.com
Transit.land: drew@interline.io