I will present our recent results on ultracold Fermi gases with tunable interactions. In a gas of Li-6 atoms, we have observed `second sound', which is a striking manifestation of the two-component nature of a superfluid. Second sound corresponds to an entropy wave, where the superfluid and the non-superfluid components oscillate in opposite phase, different from ordinary sound (`first sound'), where they oscillate in phase. Our measurements of the second-sound speed allow us to extract the temperature dependence of the superfluid fraction, which in strongly interacting quantum gases has been an inaccessible quantity so far. In a mixture of Li-6 and K-40, we have studied atom-dimer interactions by radio-frequency spectroscopy. We have observed an attraction on the repulsive side of the Feshbach resonance, which represents a unique consequence of a few-body phenomenon and only occurs in systems with mass imbalance.