  Using the knot Floer homology filtration, we define invariants associated to
a knot in a three-manifold possessing non-vanishing Floer co(homology) classes.
In the case of the Ozsvath-Szabo contact invariant we obtain an invariant of
knots in a contact three-manifold. This invariant provides an upper bound for
the Thurston-Bennequin plus rotation number of any Legendrian realization of
the knot. We use it to demonstrate the first systematic construction of prime
knots in contact manifolds other than the three-sphere with negative maximal
Thurston-Bennequin invariant. Perhaps more interesting, our invariant provides
a criterion for an open book to induce a tight contact structure. A corollary
is that if a manifold possesses contact structures with distinct non-vanishing
Ozsvath-Szabo invariants, then any fibered knot can realize the classical
Eliashberg-Bennequin bound in at most one of these contact structures.
