The USGS archive holds data collected by the Landsat suite of satellites, beginning with Landsat 1 in 1972. All Landsat scenes in the USGS EROS archive are available for download at no charge. The Search and Download page describes how to find Landsat data.
The USGS Landsat Global Archive provides a graphic view of the acquisition of Landsat data from 1972 to 2011.
Currently, Landsat 7 continues to collect data, following a near-polar, sun-synchronous orbit on the Worldwide Reference System.
The Landsat Acquisition page lists daily capture schedules, acquisition calendars, and pending acquisitions.
Landsat 7 collects about 300 scenes per day and scenes downlinked to USGS EROS are generally available to order within 6 hours of acquisition.
The collection of Landsat 5 data has not been as consistent throughout its 27 years, and as of November 2011, collections have been suspended. The latest details about Landsat 5 can be found at http://www.usgs.gov/newsroom/article.asp?ID=3231#.UGCu4vU-c7x.