Most users know about it (or should know about it), therefore, as a result it sometimes gets forgotten. Think of the seamless data distribution system as the grand-daddy of the USGS geospatial repositories... and then some! This site has complete coverage of theNational Elevation Dataset that supercedes Digital Elevation Models, complete DOQQ coverage, over 30 TB of urban imagery, one-time coverage of full resolution Landsat imagery, and much more. Also, the data are seamless, so you can put a box aroundthe data without having to figure out quadrangles and then having to assemble them all. Here's some details from our friends at the USGS if you need to hear more...

Free downloads are availble - up to 1.6 Gbytes of data in 100 MB files in one user request. Users have unlimited access to the interactive map and can make unlimited requests.The amount of coverage area depends on the number of products selected for download. For example, if NED 1 Arc Second is selected as the only product a user can download up to a 30 square degree area. But, if NED 1 arc second and NED 1/3 arc second are selected, then the area available for download will be considerably smaller. To see the various sizes of coverage area for each product, click on the List of Products drop down, select the desired product. User can define an area of interest by drawing a box, defining coordinates, or useavailable templates to cut out the area. The first method of drawing a box (square or rectangle) is best for defining ageneral area. The coordinates are displayed, in decimal degrees, in the bottom left corner of the window as the mouse is moved over the map. The second method of defining coordinates can be used when the user wants a precise area and knows the corner coordinates. The third method is the use of provided templates, such as 7.5min quadrangles. Bookmark this one, or maybe even set it as your home-page! http://seamless.usgs.gov/ - please note, the server distributed US and International data products!