The anharmonic vibrational frequencies of H2O, Cl-H2O and (H2O)(2) are calculated using potential energy surfaces computed from DFT, specifically the generalized-gradient approximation (GGA) functional HCTH and the hybrid functional B97(2c). HCTH gives reasonable agreement with experiment and can be recommended in situations where the use of a hybrid functional would be difficult. B97(2c) is found to be superior in accuracy to all other functionals tested and should be the functional of choice when the anharmonic potential energy surfaces of polyatomic systems are required for spectroscopic or similar applications. This point is illustrated by the excellent agreement with experiment obtained in calculations on formic and acetic acid using the B97(2c) functional.