[whpalmer4]

The most convenient way to do it at present is with the splendid "Where in OF" script written by RobTrew. Here's a search you might use:

projects where ([Active]) and (number of available tasks > 0)

Note that this builds a new window with just the results selected (which gets you the display you want) but it is not dynamically updated. If you completed the last available task in a project in the window, the project would continue to be displayed until you ran the search again. Similarly, a project which after modification newly met the search parameters would not appear until the next search. In other words, it may not be suitable for making a perspective unless you are willing to rebuild the perspective frequently, or can cope with it being inaccurate at times.