The churches in Megaton and Rivet City
Both churches in the Capital Wasteland, Saint Monica's Church and The Church of the Children of Atom, accept donations to their cause in the form of worldly currency. In exchange, you will receive an equal amount of good karma.
To donate, talk to Confessor Cromwell in Megaton or Father Clifford in Rivet City. Cromwell can be found standing in front of the atomic bomb in the town center and Clifford is in his church on the second floor of Rivet City just past the entrance to the science lab. One can give donations in values of 10, 50, or 100 caps as often as one chooses.