NASA’s Earth Observatory has been around for some time (at least in terms of Internet time) and is a great resource for remote sensing and mapping images. The Daily Images section is interesting enough on its own and include such recent items as a map of sulfur dioxide emissions of the Soufriere Hills volcano on Montserrat, and before and after satellite images of the Three Gorges Dam. Images and maps are usually topical and come with an interesting explanation.
Other features of the site include a table listing available data sets according to month. Data sets include sea surface temperatures, rainfall and chlorophyll. Unfortunately, there are only small world maps of each dataset on the site.

On a similar theme but updated less frequently is NASA’’s Visible Earth. It provides various thematic and remote sensing images of the planet. This site also provides detailed explanations of the imagery but provides higher resolution images. Images here can be searched by sensor or theme.