Blanch sugar snaps in a 4-quart pot of boiling salted water , uncovered, 1 minute. Drain in a colander. Immerse colander with sugar snaps in a large bowl of ice and cold water to stop cooking, about 1 minute. Drain again well, then transfer to paper towels and pat dry.
Cook shallot in butter in a 10-inch heavy skillet over moderate heat, stirring occasionally, until softened, about 1 minute. Increase heat to high, then add sugar snaps and sauté, stirring occasionally, until sugar snaps are crisp-tender, 2 to 4 minutes. Add tarragon, zest, salt, and pepper and toss until combined well.
