RTG
Radioisotope Thermoelectric Generators (RTGs)
RTGs convert the heat produced by radioactive decay of radioactive elements such as Plutonium-238(Pu-238) isotope into electrical power. As plutonium-238 decays into Uranium-234, it gives off huge amounts of heat, enough to be harnessed into electric energy, RTGs typically have incredibly low efficiencies converting thermal energy to electrical - around 5%. This means that small changes in efficiency can have a large impact on the mass/power ratio of the device.
RTGs have long been used by NASA and the Russians particularly with spacecraft when going out far from the sun when Solar panels become far less useful as a power source.
The Apollo Lunar Landers had an RTG on.
Pioneer 10 & 11 and various other spacecraft before them, and New Horizons and many other spacecraft after them, are powered using nuclear fission.
The amazing Voyagers 1 and 2 used them and they are still operating although giving a lot less power so most onboard instruments are now turned off, they will hopefully hold up for ten more years.
Chinese Chang'e 3 Lunar Rover used an RTG.
americium-241 RTGs
For low power use, americium-241(241Am) RTGs could be used but are less useful as americium-241 half-life is ~432 years versus ~88 years for 238Pu giving 1/5 of the power for the same mass. Another downside of americium-241 is that it produces more penetrating radiation through decay chain products than 238Pu and therefore needs more shielding(meaning more weight), Note Americium-241 has a very weak gamma byproduct but is safe enough to be used in many household smoke detectors,
The big upside is that americium-241 can be sourced from reprocessing nuclear waste from power generation and doesn't have to be "made to order" in a special reactor. Britain's National Nuclear Laboratory (NNL) make 10We batteries using 2kg of americium-241 using Sellafield's large store of waste plutonium. Americium-241 RTGs could be good for interstellar missions as it could hypothetically power a device for centuries, or for much longer time use batteries for probes to kuiper belt destinations beyond Pluto.