National Geophysical Data Center
The National Geophysical Data Center (NGDC) (part of the National Oceanic and Atmospheric Administration in the US) provides scientific stewardship, products, and services for geophysical data from the Sun to the Earth and Earth's sea floor and solid earth environment, including Earth observations from space.
The NGDC provides a lot of free downloadable data, ranging from gravity and magnetic to GIS data and coastlines. To get a list of all data available check the Available Data Types page of the NGDC. The department already has downloaded parts of this large open database, så before downloading check whether the data already exists on disk. You can either contact Michel Heeremans or someone in your own group for advice.