US source, with directions

• https://viewer.nationalmap.gov/basic/
• http://earthexplorer.usgs.gov/ 
• http://www.opentopography.org/
• http://coast.noaa.gov/dataviewer/?#

The following countries have good, free lidar:

• Denmark, Finland, Netherlands, Norway, Slovenia, Spain, United Kingdom

NAIP and higher resolution download  (check both USGS sites; they might have different imagery)

• Earth Explorer directions and EarthExplorer download (suggested first choice, as it has a good projection and a selection of imagery dates)
• National Map directions and National Map Viewer downloads

WMS service

• USDA NAIP web service

• SRTM30
• ETOPO1/2/5

• SRTM
• MERIT DEM
• TANDEM-X 90 m

• Landsat
• MrSid Geocover 2000

• NASA  Blue Marble next generation
• MODIS rapid response system for moderately sized areas
• NASA Worldview

• Landsat
• MrSid Geocover 2000
• Blue Marble next generation
• MODIS rapid response system for moderately sized areas

• Natural Earth
• GADM--Global Administrative Areas
• OpenStreetMap--not all countries available; insure you get shapefiles
• HydroSHEDS--15 sec resolution rivers and drainage basins
• Global roads: https://ciesin.columbia.edu/confluence/display/roads/Global+Roads+Data, http://sedac.ciesin.columbia.edu/data/collection/groads (get shapefiles)

• Global GLC2000 (1 km)
• GlobCover 2009 and 2006 (300 m)
• Corine Land cover 2000 and 2006 for Europe (100m and 250m)