The future is now. GIS forerunner company ESRI has recently published much of their geospatial analysis capability online… for free. Implementing the philosophy that knowledge is power and that all peoples and nations should be empowered to make smart and responsible decisions, ESRI is seeking to change the world by making powerful GIS tools available to anyone with web access.
-And they’ve included not only the tools, but the data as well. Called “base layers,” this data is literally something you can add to a map with a click – like roads, topography, vegetation, weather… you name it.
For only the mildest example of what they’re doing, check out a map of the Nuclear Rocket Development Station’s Test Cell C.
Explore, play around with it, create your own map web apps… get creative. With this kind of power at your fingertips, from checking out whether or not your house is on a floodplain to investigating political demographics in your area, there is literally no limit to what you can do with this.
Amazing.
Chronicles of an Atomic Astrowright 
The new ESRI online mapping program is very nice and like much of ESRI’s work, is high quality.
There are lot of good online mapping programs out there you just have to look. many counties across the States, have some form of online mapping and as do many universities. This is a similar online mapping program from my home state that also allows you download the datasets; it also has national data. http://cares.missouri.edu/default.aspx
If you want to explore the world of GIS without spending several thousand dollars on software, Quantum GIS, a open-source GIS program is available for free. It is not nearly as functional or ‘neat’ as ArcGIS or MapInfo but still does some good work and is the software suite I used when I first developed our GIS program at my workplace. http://www.qgis.org/