Often when launching deep-space probes, scientists will "loop" them around planets, "stealing" some of the planet's momentum and transferring it to the probe - giving the probe a huge speed boost.
I've heard of multiple assists being chained together, for example Jupiter, then Saturn, then flying off at enormous speed out of the solar system.
It seems that getting more and more of these assists would be beneficial to the probe's mission. Which probe that we have launched has received the most gravitational assists?