The Triangle, which must be found (or Stolen) is basically a Staff that you can Attack with. It's not ranged like the Bow & Arrows, but it dishes out a lot more damage. This is the best Weapon in the game for characters who like to make heavy use of Magic Missile, since it offers the maximum damage bonus and allows you to Attack physically without changing Weapons.
The Triangle, which must be found (or stolen) before reaching Level 7, is a very nice Weapon for Magic Missile users. On the overworld, it is basically an improved Staff—your Magic Missiles deal 2 more damage than they would with the Staff, and you can't Attack. However in dungeons where the Magic Missile boost doesn't work, the Triangle is unlike other magic boosting items in that you can use it to Attack. It's probably still better to use some other Weapons, such as a Blaster, but it's nice nonetheless.