ESRI's ArcGIS Online.
ArcGIS Online is an expensive product for anyone that is not in a K-12 school. If you work in or attend a K-12 school then ArcGIS Online is free through ESRI's ConnectED program. Click here to apply for a free subscription.
ArcGIS Online provides students with the ability to create mash-ups of geographic data on a map. Students can add data layers to the whole map or to a section of a map. Students can use data that they select from the ArcGIS library or upload their own data files.
Applications for Education
Take a look at these five lesson plans to get a sense of the possibilities that ArcGIS Online offers. Each of the lessons can be conducted without the need to register for an ArcGIS Online account. After looking at those five lesson plans you can jump into AGO Skillbuilder to find more than 70 progressively more challenging activities for students. The ESRI blog also contains some great example of using GIS in the classroom.