Autodesk Labs has a new plugin for Revit MEP 2011/2012 called “ASHRAE Viewer for Revit MEP”.

Description from the webpage:
Revit MEP utilizes the American Society of Heating Refrigeration and Air Conditioning Engineers (ASHRAE) Duct Fitting Database to lookup coefficients for determining pressure losses through duct fittings. This add-in provides a way to view the illustrations and the data table of available duct fittings from the database.

Download it from here: