Upload an Excel spreadsheet with ZIP/Postal Codes in one column and the associated attributes in the other and TargetMap will map it for free onto Google Maps. The end result is a pretty slick thematic map application with data values that can be displayed by hovering the mouse over a given area or clicking on a specific polygon. Maps can then be shared via Twitter, Facebook, or embedded onto a web site page.
All posted maps are accessible from TargetMap’s home page. For example, one of the leading maps currently is a map of changes in radiation in Japan after the Fukushima nuclear plant incident. You can search for maps by keywords or browse by category.
The catch on the free version is that you have to be willing to share your data and maps publicly. There is a premium version available for $195/year that allows the maps to be stored privately.