http://www.naturalearthdata.com/
Natural Earth is a public domain map dataset available at 1:10m, 1:50m, and 1:110 million scales. Featuring tightly integrated vector and raster data, with Natural Earth you can make a variety of visually pleasing, well-crafted maps with cartography or GIS software.
http://landcover.usgs.gov/landcoverdata.php#na
Great set of links to almost all land cover datasets. Links here include most of the datasets below, and many more esoteric data such as river observations, aquifers data and ocean colour information.
http://freegisdata.rtwilson.com/
Various free data sets
http://www.datapages.com/Partners/AAPGGISPublicationsCommittee/GISOpenFiles.aspx
The AAPG, under the direction and guidance of the GIS Publications Committee and AAPG Datapages, is building a GIS / spatial library comprised of peer-reviewed Global Framework, Geothematic and Geographic themed projects. The project files are free to the public and can be downloaded from this GIS Open Files page.