Could a spacecraft orbiting Saturn, like Cassini, use a Titan gravity assist to leave the Saturn system (e.g. to bring a sample back to Earth), without using fuel other than to reach Titan?
You'd need to get to Jupiter to get an assist to Earth. It might be possible. See Saturn Escape Options for Cassini Encore Missions and Cassini End-of-Life Escape Trajectories to the Outer Planets. You can certainly escape Saturn with multiple Titan flybys. Getting to Jupiter is trickier, and requires the right timing and a lot of patience. Low-energy Saturn-Jupiter transits are available about every 20 Earth years.