BoundingBox 3DView creates a 3D section box around any objects that are selected when the tool is activated.
How it works: in a model view such as a floor plan or 3D view, make a selection of objects of interest. Run the application through the Add-Ins Tab & External Tools pull down or a keyboard shortcut of your choice, and a dialog is displayed.
If the current settings meet your needs, just hit enter on your keyboard as the OK button has focus by default. If you change the bounding box margin or the 3D view options, these will be remembered next time you launch the command again (in the current Revit session). Once it finishes, open the new or modified 3D view.
Download BoundingBox 3DView here.
Thanks to Clay Hickling of GHD and his presentation at the Revit Technology Conference USA 2011, "10 Free Revit Add-ins You can't Live Without"