The National Map Seamless Server has a lot of useful GIS data. One of the best aspects is that they have ortho imagery that you can download in various formats so that you do not have to convert images if you do not have the ability to view a specific format. This is very useful to those that are using open source GIS software and do not have mr. sid viewers. Here is some links to the GIS data that is available for download.
1 meter Orthoimagery
Bureau of Transportation Statistics (BTS) Roads
Digital Raster Graphic (DRG)
High Resolution Orthoimagery
Landsat Mosaic Orthoimagery
List of High Resolution Orthoimagery Areas
LP DAAC MODIS
Modis NDVI Direct Broadcast
National Atlas
National Elevation Dataset (NED)
1 Arc Second
1/3 Arc Second
1/9 Arc Second
National LandCover Dataset (NLCD)
1992
2001
Shuttle Radar Topography Mission (SRTM)
1 Arc Second
3 Arc Second