I subscribe to a blog with weekly(ish) posts on the subject of free GIS data, and this week's post is about a new initiative by the University of Wisconsin called the Atlas of the Biosphere:
The Atlas provides access to maps and associated data related two four main themes: human impacts, land use, ecosystems, and water resources. I encourage you to take a look and see what's available.
The data sets are global scale and hence may be too coarse for some finer-scale analysis or applications, but may be useful for providing a general overview, or for filling gaps where no finer-scale data exists. It's also a great example of a simple, well-designed web site that allows free access to data - something that many of us would likely be happier to see a lot more of.
A more detailed description of the Atlas of the Biosphere is available at the above mentioned blog: