The Library of Congress has posted a preliminary schedule of concerts in the 2008-2009 series at the Coolidge Auditorium. Notable are several concerts commemorating the centennials of two giants of modern composition, Olivier Messiaen and Elliott Carter (the latter of whom is still living).
All these concerts are free of charge, but it’s generally a good idea to get tickets in advance (through Ticketmaster, unfortunately, so you pay a $4ish service charge). Still, I’ve never had a problem getting tickets at the door if I didn’t mind sitting way at the back of the auditorium. Tickets are not available yet but availability dates are listed in the schedule.