When I was the CIO of Utah, I used to quip that every data record I cared about had a SSN or latlong coordinates. There's lots of things that are geographically categorizable, but commercial GIS software is expensive and consequently, geographic Internet applications are few and far between. Even things like MapQuest don't really do much besides give you directions. Why can't I annotate them, for example? GRASS is an open source GIS system that provides raster, topological vector, image processing, and graphics production functionality. I went to a talk today by Schuyler Erle and Rich Gibson on using GRASS. Here's the slides. There are easier tools to use (like Quantum), but GRASS provides more advanced functionality using map algebra. That advanced functionality comes at a cost: GRASS isn't easy to use.