While understanding cells' responses to mechanical stimuli is seen as increasingly important for understanding cell biology, how to best measure, interpret and model cells' mechanical properties remains unclear. We determine the frequency-dependent shear modulus of cultured mammalian cells using four different methods, both novel and well established. This approach clarifies the effects of cytoskeletal heterogeneity, ATP-dependent processes and cell regional variations on the interpretation of such measurements. Our results clearly indicate two qualitatively similar but distinct mechanical responses, corresponding to the cortical and intracellular networks, each having an unusual, weak power-law form at low frequency. The two frequency dependent responses we observe are remarkably similar to those reported for a variety of cultured mammalian cells measured using different techniques, suggesting it is a useful consensus description. Finally, we discuss possible physical explanations for the observed mechanical response.