The Ansata's Inverter.
The Inverter was a power source used by the Ansata of Rutia IV for folded-space transport. The Inverter allowed the Ansata to greatly increase the effectiveness of their terrorist tactics in their campaign for independence.
When that group attacked the USS Enterprise (NCC-1701-D) when it was in orbit of the planet in 2366 the crew of that vessel was ultimately able to track the location of the power source and use it to free captured hostages Beverly Crusher and Jean-Luc Picard. (TNG: "The High Ground")