JetBlue and American Express right now are having a promotion that nets you $15 off a one-way fare when you book via this page at JetBlue.com using your American Express card. The discounts are combinable, so you can get up to $30 off on a round-trip if both flights meet the rules of the promotion.
This offer is valid for flights through Nov 14, 2007, excluding Friday and Sunday travel. The offer expires on Oct 31.
Combine this with the JetBlue triple points promotion going on right now, and you've got one heck of a great deal!
By the way, if you travel on JetBlue, there is no better deal than the JetBlue American Express card. If you signup now, you get 50 TrueBlue points, which is half of what you need for a free ticket. If you fly to say the Caribbean from New York with your new JetBlue Amex card with the triple points promo, you immediately accrue a free round-trip flight anywhere JetBlue files, plus you have 22 points leftover toward your next award ticket. Saving $30 on your ticket is just icing on the cake.