Quantum GIS is an opensource desktop GIS tool that supports ESRI Shape, MapInfo Tab/MIF, PostGIS and various raster formats such as GeoTiff, ErDas Imagine, Jpeg 2000, ERMapper (ECW), and ESRI ArcInfo ASCII Grid. It has both viewing and some rudimentary editing and attribute querying capabilities.
Quantum works on Windows and Unix. It works on Mac as well, but not quite clear how well it works on Mac.
The most recent version of QGIS has a plug-in architecture for Python to allow for creating Python based plug-ins. It also works with GRASS.