(Part of Open Source Transport Informatics tools listing, for the OSSTIP project, especially OSSTIP/WP2- Transit Informatics tools review.)
Mapnificent (http://mapnificent.net) was developed by Stefan Wehrmayer, and was "inspired by Mapumental". Whilst not as computationally complex or accurate as Mapumental, it can parse GTFS data to show travel time maps to destinations in multiple cities around the globe, and is currently free to use.
The results are integrated and displayed with Google Maps.
The first version's source code is available, whilst the developer claims later versions will be published soon.