Fallout version (left) and Fallout Tactics version (right)
Rope is a strong, thick line made of braided brahmin hair. About 45 feet in length.
In Fallout, there are three uses; two are needed to descend the elevator shafts in Vault 15, and one to enter the Glow. Ropes can be found in several places throughout the game. Quite a few merchants stock them, including Seth in Shady Sands. In addition, rope can be found in two places in Shady Sands; on a bookshelf in one of the buildings, and on the floor in the hut next to the field where the plants are growing. There is also a rope located in Vault 15 itself, to allow immediate descent to the third level. There are also three ropes on the table in the merchants room inside the Cathedral. There is another located in Junktowns shop in Killian's room on the bookcase
In Fallout 2, its only use is to climb down the well in Modoc. One can be found conveniently nearby in Farrel's garden. Three can also be found right outside the Military Base, in the locked shack in the third box to the right.