Wadsworth

 is your personal Mr. Handy butler in Megaton, which you get, together with a house, for disarming the nuclear bomb that lies in the middle of the town. He can give you instructions on how to spruce up your house, give you haircuts and tell science-related jokes (only one at a time, so he can recharge his 'humor emitter array'). Wadsworth can also give you Purified Water, but only five at a time before needing to recreate his supply. It takes a week for him to recreate his supply.

Jokes

 * 1) I was going to the clairvoyant's meeting, but it was canceled due to unforeseen events.
 * 2) It is common knowledge that irradiated cats have 18 half-lives.
 * 3) Two cannibals are eating a clown, one turns to the other and says "does this taste funny to you?".
 * 4) Photons have mass?  I didn't even know they were Catholic. (Note: Robert Langdon says this in the book Angels and Demons, the prequel to the famous book, The Da Vinci Code, except about neutrinos, not protons.
 * 5) I once visited a crematorium that gave discounts to burn victims.
 * 6) The best contraceptive for old people is nudity.
 * 7) Two atoms are sitting in a bar. One says to the other, "I think I've lost an electron." The other asks "Are you sure?" to which the first replies "I'm positive."
 * 8) A neutron walks into a bar. "How much for a drink?" To which the bartender responds, "For you, no charge."
 * 9) War does not determine who is right- only who is left. (Note: Interestingly enough, this is a quote from Bertrand Russell. This same quote also randomly appears, among others, when you are killed in Call of Duty 4.)

Appearances
 appears only in Fallout 3.