The Bobblehead - Agility is a Fallout 3 item, a Vault Boy Bobblehead. It permanently raises your character's Agility by 1, to a maximum of 10.
It can be found in the Greener Pastures Disposal Site's main office on a desk, next to a computer terminal. Careful, though. A field full of nuclear waste, a large amount of radiation, and maybe even a deathclaw or two stand in your way.
Obtaining Bobblehead Agility