The United Nations Food And Agriculture Organization (FAO) maintains the FAO GeoNetwork, a data portal for worldwide digital geographic information from the FAO, other UN agencies, NGOs and other organizations. Despite the “Food And Agriculture” in the name, there’s far more than just agricultural-related data here.
The FAO Portal Page has a web search interface for tracking down data. Not all the data is available in downloadable digital format – for much of it, only the metadata is available. If you’re only interested in digital data, just check “Digital” next to “Map type” box. Search results can be random in nature, and sometimes data listed as being available in downloadable format isn’t. But you never know what useful stuff might pop up. For example, here’s a map of the southern tip of Africa, plotted from 1:125,00 scale coastline vector data for Africa and Arabia derived from SRTM data:
Recent coastline data with that kind of resolution isn’t always easy to find.
You can subscribe to an RSS feed that lists new datasets as they’re added to the portal. There’s also an interactive online mapping system for creating your own maps, and an extensive list of WMS servers with a wide variety of information you can add to WMS-compatible programs like MapWindow and Google Earth. You may not always find exactly what you need, but it’s definitely worth a look.