I recently heard of GTFS as a specification for the times and geographic information regarding transit networks. It is initially used by Google to integrate them into Google Maps but is now made to be a standard.
I understood there are a number of applications that rely on this specification.
But I would like to know if there is another format which is more or comparably popular (regarding the widespread use) for public transit data? The intent would be to figure what format would be the most interesting to release data.