The DRG files are in GEOTIFF format, which means that georeferencing information is embedded in the TIFF header so that mapping software can determine the geographic coordinates of features. The USGS DRG files include the map collar, the area surrounding the map outside the neat line and which contains the map scale and other information. Some of the sites below have processed the images to remove the collar so that the images can be readily tiled.
In most cases, the DRG files have been reprojected from the original map projection into the UTM projection. Don't be deceived by the notation visible in the map collar which may indicate a Polyconic or other projection. The DRG files produced by the USGS are in UTM projection, with the exception of portions of California. Some of the sources below have further processed the data into other projections. Usually the site will note if additional processing has been done.
Because the images are in TIFF format, they can be viewed by most popular image viewing software. The images are large, about 7-10Mb each, and a computer with limited speed or memory may have difficulty viewing the images.
For technical information about the USGS DRG files, see the USGS site at http://topomaps.usgs.gov/drg