They are the main competitor to ArcView. MapInfo can make better looking maps, but most of our staff are more proficient with ESRI products. An experimental on-line atlas that uses GIS technology and concepts to display many different kinds of data down to the county level. Most of the data is available for free download. The terraserver allows you to search for particular locations and view aerial photos taken by USGS and satellite photos from Soviet spy satellites.
Data is available for purchase.
Most of it is government produced and non-coyrighted.