Performs cartographic transformations and geodetic computations.
The Proj class can convert from geographic (longitude,latitude) to native map projection (x,y) coordinates and vice versa, or from one map projection coordinate system directly to another.
The Geod class can perform forward and inverse geodetic, or Great Circle, computations. The forward computation involves determining latitude, longitude and back azimuth of a terminus point given the latitude and longitude of an initial point, plus azimuth and distance. The inverse computation involves determining the forward and back azimuths and distance given the latitudes and longitudes of an initial and terminus point.

The open-source multi-modal trip planner, i.e. a routing engine for street networks or potentially any other spatially enabled graph

"Almost four years on from Mapping Hacks, the state of web-based mapping is moving within reach of mortals and designers. The one piece of software most directly responsible is Mapnik, Artem Pavlenko's excellent open source rendering library. Despite a not-undeserved reputation for being difficult to install (there are even what appears to be companies offering Mapnik as a service), once you've got it running there's nothing better out there for generating your own web-based slippy maps." some notes on producing high-quality tiled maps. Interested independent of Mapnick

Python-based WMS-C/TMS tile caching server, with pluggable caching mechanisms and rendering backends.

WMS tiles cacheimplemented in Java

pure JavaScript library for displaying map data in most modern web browsers, with no server-side dependencies. In addition to supporting image-based rendering, adds support for vector overlays as well. Extensible layer API for supporting different data source, includes data out of KML and GeoRSS streams.

Java vector GIS viewer, reads out of PostGIS, WMS, and Others. Open Source