Two courses from the Philadelphia area that deserve mention are Jeffersonville and Paxon Hollow. These certainly are not sleepers in the strongest sense of the word. They have been discussed on here in the past and, as municipal courses in densely populated areas, get considerable play. They are sleepers in that their architecture is well beyond that of the typical muni at the $30 - $40 price point, and I think that quality is underappreciated by the majority.
Paxon Hollow was constructed in 1926, and boasts a few very strong holes. Two holes of note are #10, which is a drivable par 4 with a stream protecting the green. Eighteen is a great finishing hole. The hole plays 535, uphill all the way with the same stream from #10 in play on the tee shot. Both holes have been discussed in this DG before. In answer to the question that started the thread, this is my favorite sleeper.
Jeffersonville is a Donald Ross design which opened in 1931, and was restored in 2002 by Ron Prichard. Overall, great conditions and great strategy in place through all 18 holes.