Freely available remote sensing data and GIS datasets

One of the biggest issues with setting up a spatial project is to get base data to work with. I thought it would be helpful for forum members if we could list good sources of freely available datasets, I will start by listing some of the best links that I have found.

Landsat 8 Data from the USGS.

Land cover data including elevation from Global Land Cover Facility:

An extensive listing of free GIS data covering most countries by Robin Wilson: