The Ninja perk grants you the power of the fabled shadow warriors. When attacking, you gain a +15% critical chance on all Melee and Unarmed attacks. Additionally, Sneak attack criticals do 25% more damage than normal.
Notes
To attain this perk you are only required to have an 80 Melee Weapons skill, even though the perk affects the critical bonus and sneak damage of both Unarmed and Melee Weapons.
Bugs
PCPlayStation 3Xbox 360 In Fallout: New Vegas, Ninja multiplies your critical chance by 1.15 instead of adding a flat 15%, unlike all other perks/traits that modify critical chance. For example, a character with a critical hit chance of even 25% would have it be increased to only 28.5% rather than to 40% as it should. (25% * 1.15 = 28.75% vs 25% + 15% = 40%)