A new categorised list of freely available GIS datasets is available at http://freegisdata.rtwilson.com/. It includes both world-wide data and country-specific data, and is sorted by category (such as Natural Disasters, Land Cover, Hydrology etc).
All the datasets linked to on the site are free for academic and/or non-commercial use, some are also free for commercial use.
The datasets have been gathered from a number of internet sites, from discussions on forums such as LinkedIn and from personal email contact with data developers. Of course, the site is nowhere near finished yet - and I would welcome any extra links for free GIS data to add to the site.
Please see http://freegisdata.rtwilson.com for more details, or my personal website at www.rtwilson.com/academic