The Google Transit Feed Specification is used by several non-Google applications:

• TimeTablePublisher can take GTFS feeds and format them into static schedule tables for online and print applications.
• Graphserver is an open-source transit (and more) routing engine, with support for Google Transit Feed Spec feeds
• GIRO's HASTUS transit scheduling software has a GTFS export option.
• Trapeze offers a GTFS export tool for their Trapeze FX schedule management software.
• Next Insight Transportation Software offers a Google Transit Feed Creator
• Trillum Solutions (formally Transit Info Solutions) offers custom-designed transit websites and a web-based schedule manager with Google Transit export
• Merakas is a Lithuanian company whose PikasPHP transit software can read and write GTFS. (An example deployment for Vilnius, Lithuania can be seen here.)
• Avego Future Fleet has announced GTFS import and export for free

See also the GTFS Consumers, GTFS Exporters and GTFS Services categories at http://headwayblog.com/wiki.