This database is designed for discovery of free GIS data on a wide variety of topics. As it is a collaborative project of the Big Ten Academic Alliance, there is an emphasis on data from the Midwest United States.
The Harvard Dataverse repository provides a searchable database of researcher uploaded content. It is not exclusively GIS data, but does have GIS datasets. Be warned that this is an open forum and not all data is reviewed or credible.
User forum map making and for sharing maps and data. You will need to login with an MSU NetID to make an account (please see the Accessing ESRI ArcGIS Software page for instructions). Be warned that this is an open forum and not all data is reviewed or credible.