There is a guy in Paradise Falls that walks up to you and gives you a random item. These items can be ammo, stimpacks, food, nuka cola, and caps. Once you know what he looks like (not by what armor he wears, b/c that changes, but facially), you can just walk up to him and keep talking to him for free stuff. If you can't find him inside, he is usually in the barracks. I got about 200-500 10mm rounds and shells, about 1000 5.56mm rounds, 100+ stimpacks, and alot of food in about 20-30 mintes.
The downside to this glitch(if it is indeed a glitch) is that you have to be evil to get into Paradise Falls, and that it is a lengthy process to get useful results.
(supplied by: Angus_Young_Jr)