You're legally limited to 2.5 inches (6.25 cm)blade length in Boston.
Boston Municipal Code of Ordinances
16-45 PROHIBITING THE CARRYING OF KNIVES OR SIMILAR WEAPONS.
16-45.1 Carrying of Weapons Prohibited.
No person, except as provided by law, shall carry on his person, or carry under his control in a vehicle, any knife having any type of blade in excess of two and one-half
2½ inches
If you've got the time and money get one of FrontSight's HideAway knives.
http://www.hideawayknife.com/main.php They're quite a nasty little package.
If you're limited on time and money, you can get the CRKT Fixed Falcon, Bear Claw at 6.0 cm, Camillus Mini Talon at 5.6 cm, Emerson LaGriffe at 4.4 cm, or Outdoor Edge Wedge at 5.6 cm