The OnEarth/OnMars Web Map Service (WMS) offers a number of very large image datasets to many WMS clients. Information about these datasets as well as a basic viewer can be found at http://onearth.jpl.nasa.gov/ and http://onmars.jpl.nasa.gov/
The WMS server URLs required by most WMS clients, are found at OneEarth and Onmars.
Many third party applications can make use of these services. Some examples include: