US has put shuttle flights on sale between BOS and LGA or DCA and LGA.

$78 RT, same day Saturday travel only, applies on December 1, 8, 15, 22 and 29, 2001
$118 RT, travel 10 am - 2 pm or 7 - 10 pm Monday - Friday, or all day Saturday and Sunday

http://www.usairways.com/promotions/specials/shuttle_promo_fares.htm