Plutonium gives off lots of thermal energy. For example, more than one third of the energy produced in many nuclear power plants is from Plutonium. It is/ was also used as a power source on space probes like the Galileo, Voyager, and New Horizons along with the Curiosity Mars rover.
MOX Fuel
Mixed Oxide fuel is fuel used in light water reactors. It has 60 kg of Pu for each ton of fuel. MOX fuel can be used in thermal reactors and is currently used a lot in Europe and Japan.
Medical Advances with Plutonium
Plutonium can be used to power artificial heart pacemakers which stimulate regular heartbeats when people's bodies can't. It isn't a common power source to pacemakers anymore, but was commonly used back in the 1970's.
Medical Experimentation
During World War II Plutonium was used studied and tested on animal and unwilling human subjects. The Experiments were conducted by scientists working on the Manhattan Project.
Nuclear Weapons
Plutonium can be used in atomic bombs and was previously used in the Fat Man and Trinity bombs which had a Plutonium core.
Alloys
Plutonium alloys have many uses. For example, Plutonium-aluminium, Plutonium-cerium, etc. can be used in nuclear fuels. Also, Plutonium-gallium- cobalt can be used as a superconductor.