GRASS (Geographic Resources Analysis Support System) is a powerful, open-source desktop GIS application that supports "geospatial data management and analysis, image processing, graphics/maps production, spatial modeling, and visualization". Although the standard GRASS installation includes a graphical interface, Quantum GIS  can serve as a more user-friendly interface via its GRASS plugin.
Online tutorials and demos
- GRASS GIS in a Nutshell , a great way to start learning GRASS
- An Introduction to the Practical Use of GRASS 6.0 , a comprehensive guide for new users
- Markus Netler's GRASS GIS Seminar  (November 2006)
- GRASS/R kriging example  from UC Davis California Soils Resource Lab
- GIS Concepts