- FeedValidator - Google supported Python-based GTFS validator.
- gtfs-validator - A GTFS validator based on the Onebusaway GTFS Modules, runs in Java and is faster than the Google provided one. A fork was also made off of this validator offering additional checks
- GFTS format with schemas, a data package and tests - From Stephen Gates, a data package specification with validation accomplished with Good Tables.
Original content borrowed from awesome-transit