<p>I'm researching Open Source GIS resources - and wonder out of all the Open Source tools that are out there - what people are actually using / finding useful.</p>

<p><a href="http://en.wikipedia.org/wiki/GIS_software" rel="noreferrer">http://en.wikipedia.org/wiki/GIS_software</a></p>

Here's a few I use regularly
postgis
gdal
mapserver
uDig
grass
geos
geoserver
gmt
openlayers
tilecache
qgis
Big list here
http://opensourcegis.org/