Proj4js is a port of the Proj.4 (and gctpc) C Libraries into JavaScript for conversion of point coordinates from one coordinate system to another, for use in web mapping clients.
Provides methods for coordinate transformations between map projections and longitude/latitude, including datum transformations.
Initialization of Proj objects is with a projection code, usually EPSG codes.
The Subversion repository for the existing codebase is here. Discussions related to this project take place on the mapbuilder-proj mailing list.