Across the years, I've been occasionally in need for geospatial data, whether a World Map, or a Satellite backdrop to a small map for a city, a free satellite image or shapefile was very important.
Luckily, many data sources exist in the Internet that have been published by many Space and satellite-owning agencies across the world, and many derived data that can enhance any map.
Some of my more important sources:
Global Land Cover Facility (GLCF)
http://glcf.umiacs.umd.edu/
A great source for many satellite imagery products and datasets, it can be accessed through ftp and provide wealth of file formats and resolutions, must check it.
NASA LandSat GeoCover data set
https://zulu.ssc.nasa.gov/mrsid/mrsid.pl
It provides access to LandSat Mosaics that has been created for most of the globe in 2 time eras (1990 and 2000), it delivers MrSID compressed mosaics with 15m pixel size.
StatSilk List of downloadable free shapefiles
http://www.statsilk.com/maps/download-free-shapefile-maps
Provides a list of downloadable shapefiles that contains many datasets.
Global Administrative Areas
http://www.gadm.org/
Provides shapefiles for administrative boundaries for the countries.
Blue Marble Next Generation
http://earthobservatory.nasa.gov/Features/BlueMarble/
(Direct download) ftp://neo.sci.gsfc.nasa.gov/bluemarble/bmng/bmng_arcview/
The Blue Marble is a gorgeous dataset, it provides 500m imagery for the whole world that is color processed and provides a great basemap backdrop for World Maps.
True Marble (by Unearthed Outdoors)
http://www.unearthedoutdoors.net/global_data/true_marble/download
Another global dataset that is created from LandSat imagery, this time the data has been color and band processed to provide a more natural color and appearance of the imagery, but the full resolution 15m imagery are available for purchase only, in the link you can download the the 250m images.
Global Land Cover 2000 (by the EU JRC)
http://bioval.jrc.ec.europa.eu/products/glc2000/glc2000.php
The Global Land Cover objective is to become a harmonized land cover database over the whole globe, it provides Land Cover maps for the whole world and can be utilized in many analysis functions (mostly on the national level).
