(Part of Open Source Transport Informatics tools listing, for the OSSTIP project.)
Started in 2004 by MIT Researcher Bradley Martin-Anderson, GraphServer is an Open Source multi-modal public transport trip router, written in Python.
One of the uses of GraphServer was providing the back-end computations as part of the Transit Time Maps visual application on the Walk Score website.
Much of the core capability of Graphserver was also ported into the OpenTripPlanner tool.