Water Beggars

Water Beggars is a repeatable quest in Fallout 3.

Walkthrough
Pure water is a necessity in the Capital Wasteland, but a lot of wastelanders don't have access to it and can't get into settlements that provide the water for free. Three wastelanders in particular (Micky, Carlos and Willy) can be found sitting outside of Megaton, Rivet City and Tenpenny Tower, respectively, begging for water.

Giving them purified water (or non-FEV modified Aqua Pura) gives you positive Karma (+50). Denying them water causes them to die at the next map change, even if they were given purified water before. They do not die if you do not explicitly tell them to find their own water, however, and their death incurs no Karma loss for you.