Square Enix released a few download weapons for Final Fantasy XIII-2 today. Calamity Blade (Noel), Genji Bow (Serah) and Muramasa (Noel) are available for ¥100 each.
These weapons give your character's special property boosts. Calamity Blade and Genji Bow increases the speed of your ATB gauge. Muramasa ups your chain bonus.
You'll remember the Calamity Blade name from one of the game's retail-specific bonus items.