Archives are also available from Temis: archive.
Other data of interest, on atmosphere conditions, such as ozone column and aerosol optical depth are also available from Temis.
Landsat images are available at Landsat Missions web site of the USGS.
And many of them are downloadable through a web interface at Web-enabled Landsat data (WELD) .
Images European satellites and also from Landsat missions are available through esa’s Earth Observation site at Earth Online site.