Crossbow
The Crossbow is a long awaited weapon for the Assassin's Creed series. This weapon was originally seen in the trailer for Assassin's Creed, but was not included in the game due to historical inaccuracies.
There is also hinted to be a crossbow in the upcoming Assassin's Creed: Brotherhood as one is seen on Ezio Auditore da Firenze's back in a concept art, and at the end of the E3 singleplayer trailer. Also in the Brotherhood E3 2010 Story walkthrough, it was said that during horse rides you could use weapons such as the crossbow.
Since it is a weapon that requires ammo, arrows and darts are suspected to be purchasable at the Blacksmith. It is unknown what is the maximum of ammo for the Crossbow or if the amount could be maximized, possibly by upgrading pouches at the Tailor.