The Trench Knife is a Melee Weapon in Fallout 3. It's a unique variation of the Combat Knife that is added to the games first DLC Pack Operation: Anchorage. It deals the same amount of damage as the standard Combat Knife.
The Knife can be found in the Armory at the Outcast's Outpost after you complete the simulation. It can be repaired with regular Combat Knifes.