From my hometown Urbana, vector and raster data for all the world!
"Access Illinois GIS data sets and documentation (metadata) for ArcIMS Interactive Map Services, USGS digital topographic maps, aerial photography, orthoimagery, orthophotography, geology, land use, natural resources, and infrastructure. Illinois data sets and map data layers are available for download free of charge."
A collection of tools and links to open source geo-stuff. Comes with helpful descriptions. *Beware of some faulty or broken links. If you encounter this problem, usually "google"-ing the link will bring you to the correct site.*