What is GIS?

A geographic information system (GIS) is a computer system for capturing, storing, checking, and displaying data related to positions on Earth's surfaces. GIS can show many different kinds of data on one map, such as streets, buildings, and vegetation. This enables people to more easily see, analyze, and understand patterns and relationships. (cited from National Geographic Society)

QGIS - a free and open source geographic information system
gvSIG - the largest online free geomatics professionals
Whitebox GAT - an open access GIS and remote sensing software package
SAGA GIS - a tool offering a fast growing set of geoscientific methods
Working With GIS data? Librarians Can Help With That - GIS resources prepared by the School of Information Studies, Syracuse University