OpenEV: A high performance raster/vector desktop data viewer and analysis tool
GDAL/OGR: A library and set of commandline utility applications for reading and writing a variety of geospatial raster (GDAL) and vector (OGR) formats
PROJ.4: A cartographic projections library with commandline utilities.
OGDI: a multi-format raster and vector reading technology noteworthy for inclusion of support for various military formats including VPF (ie. VMAP, VITD), RPF (ie. CADRG, CIB), and ADRG.
Python: a scripting language.
Grass 6.0 – the granddaddy of GIS apps – Grass Home Page
