Independence Air is offering flights from $29 each way for bookings through August 30th. First-time roundtrip bookings also receive $25 off when joinging their frequent flyer program.
DC to Pittsburgh is the cheapest at $29 each way. Flights betwen DC and New York City, Greensboro or Raleigh/Durham are $44 each way. Flights between DC and Detroit, Syracuse, Knoxville or Indianapolis are $54 each way.
Other major carriers are selectively matching, though look for more restrictive rules on their tickets.