The University of Arkansas Spatial Library is a massive (multi-terabyte) Oracle database holding a wide range of geospatial data sets in an OGC-compliant interoperable form. See
http://www.cast.uark.edu/UASL/ or jump directly to
http://www.cast.uark.edu/UASL/raw_DOQ/index.htmlDatasets include DRG, DOQ and landsat imagery

University of Arkansas Spatial Library