GAIA3 - A nice Geospatial Platform

Gaia is a platform designed for advanced geospatial network and SDI needs.

It is available for Windows as well as Linux and Mac (on Mono).

Based on the CarbonTools PRO open-geospatial development toolkit, this viewer can access an array of geospatial sources such as the Open Geospatial Consortium (OGC) Web Mapping Service (WMS), Web Map Tile Service (WMTS), Web Coverage Service (WCS), Web Feature Service (WFS), and Filter Encoding (FE), services such as Microsoft Bing Maps, Yahoo! Maps and OpenStreetMap (OSM), as well as file formats such as ESRI Shapefiles, Google Earth KML/KMZ, DXF, MIF, Geography Markup Language (GML) and GML Simple Features (GMLsf).

More info: http://www.thecarbonproject.com/gaia.php 
Intro video: http://www.youtube.com/watch?v=ZwtS9SZXNDk
User guide: http://www.thecarbonportal.net/downloads/Gaia/ver.3.4/Gaia3_4_UserGuide.pdf
Source code: http://www.thecarbonproject.com/dev.php