"Most of the 100+ available products are updated within three hours of observation, essentially showing the entire Earth as it looks 'right now'. This supports time-critical application areas such as wildfire management, air quality measurements, and flood monitoring."- NASA Worldview WebsiteOf note, browsing on a tablet or smartphone is supported. There is a layered slider for time, so users can choose the year, month, and day. Selecting dates in some websites can be painful. However, this slider is intuitive, quick, and easy to use--much more so than those pesky calendar pages some users are forced to navigate. Users also have several base maps to choose from in addition to the rest of the layers. There is a great "About" page and brief "Tour" available.
|Global Land Surface Temperatures/Day from MODIS, 17 July 2013|
You can also check out recent scenes from Landsat 7 at: http://earthnow.usgs.gov/ or Landsat 8 at: http://landsatlook.usgs.gov/. If you are interested in NASA's open data policies, a quick overview can be found at: http://science.nasa.gov/earth-science/earth-science-data/data-information-policy/