The knot Floer homology is a very powerful Floer theoretic invariant of knots in 3-manifolds which was recently introduced by Ozsvath and Szabo, also independently by Rasmussen. It detects the genus of a knot and whether a given knot is fibred. It also provides lower bounds for the four ball genera of knots in the three-sphere. The purpose of this talk is to discuss yet another application of the knot Floer homology in low-dimensional topology. Using the underlying filtered chain complex of the knot Floer homology, we will calculate some invariants of contact manifolds that are obtained by surgery on a Legendrian knot. Specifically, we will calculate a numerical invariant that is monotone under Stein cobordisms and a Floer cohomology class that gives an obstruction to overtwistedness of contact manifolds