Using MassGIS mapservices:
The front page of the MassGIS' ArcGIS Online site is a horizontally scrolling page of content (services, applications, documents):
Clicking on one of these items of content provides more information. For example clicking on the mapservices "Digital Globe 2011-12 Ortho Image Base Map" will open the mapservices in the ArcGIS Online web map:
Clicking on the blue link "More Details" will bring up a metadata page for the content item:
Some MassGIS Layers (mapservices) can be opened in ArcGIS Explorer Online. At this time none can be opened in ArcGIS for Desktop (ArcMap).