This morning we came across a cool dataset that none of us had seen before. The NASA Jet Propulsion Lab have made available a global mosaic that is updated daily (yes, daily!). It is only 250m resolution, but did I mention that it was updated daily?
There is a link to a web viewer here: http://onearth.jpl.nasa.gov/
The data is also available as a WMS from here http://wms.jpl.nasa.gov/wms.cgi or as KML from here http://onearth.jpl.nasa.gov/OnEarth_DailyPlanet.kml.