The United States Geographic Survey (USGS) has quietly launched the Digital Map--beta, a "a new kind of georeferenced map that is a synthesis and evolution of USGS's legacy digital map data files, the Digital Raster Graphics (DRG)." According to Adena Schutzberg at the All Points Blog, the beta was released on May 8, 2009.

From the site, "The initial version of "Digital Maps – Beta" includes orthoimagery plus roads and geographic names in the traditional 7.5 minute quadrangle format. The "Digital Map - Beta" is easily accessible, has reasonable file sizes and a common format with embedded tools, and theme-based layers."

A quick start guide is available in pdf format. 

While the site is still in beta, there may be some variability in its functioning, but this is a fantastic step toward presenting USGS mapping online.