Imagery from SPOT satellites 1 through 5 has been offline for a year or so, languishing on a server. Now the French Space Agency is making them available for free on the SPOT World Heritage website. The archive covers the full life span of the five satellites, from 1986 to 2015. SPOT 1-5 were some of the few satellites imaging at that time, and so they offer a wealth of information for studying our changing Earth. The resolution varies from 20-meters to 2.5-meters. Imagery is delivered in level L1A format with radiometric corrections applied but without geometric corrections. You can check out all the specifications here as well as the imagery license here.