The U.S. Geological Survey will release all Landsat data free of charge over the next several months. The Survey will start placing current Landsat 7 data in an archive by July. The data will be processed automatically to a standard recipe and made available for download. Landsat data from earlier time periods will be added through January 2009, when the earliest frames from 1972 will be added to the archive.
The USGS Technical Announcement gives more details. Landsat scenes can be previewed and downloaded using the USGS Global Visualization Viewer at http://glovis.usgs.gov. Under “Select Collection” click Landsat archive: L7 SLC-off (2003-present).
Landsat imagery is a 35-year record of the earth's surface, useful in applications from climate change science to emergency response.