JetBlue is offering an unlimited one-month pass (September 8-October 8) for $599.  It's a great deal, with only a few fine print points:

• International and Puerto Rico taxes not included
• Each flight Reservation must be confirmed 3 days in advance
• A $100 cancellation/change fee applies to all flights changed/canceled/no-showed within 3 days of departure
• 35 True Blue frequent flyer points are earned for purchase, no additional points are awarded
• The pass must be purchased by August 21, or while supplies last.

Full Terms and Conditions here.