Free Spatial Data
Download country level data for any country in the world: administrative boundaries, roads, railroads, altitude, land cover, population density.
A new file with the (2011) global country boundaries
Global climate data
Species occurrence data
Crop (genebank) collection data
Near global 90 meter resolution elevation data
High resolution satellite images (LandSat)
for nearly all of the world can be downloaded here. They are in the MrSid format that can be visualized in DIVA-GIS (note: they are in UTM projections, so you will need to project your shapefiles to UTM as well).
A very good list
of data sources from the Eden project.