The USGS recently issued a press release announcing the availability of Landsat data through two USGS web sites:
As of June 4, 2007, the USGS will be releasing selected Landsat 7 image data of the United States through the Web (glovis.usgs.gov or earthexplorer.usgs.gov). These data are of high quality with limited cloud cover.
The move is part of the Landsat Data Continuity Mission (LDCM) in which user feedback will help define future distribution systems for the satellite imagery.