What are some sources of unconventional nuclear power?

Nuclear power is typically divided into two categories: fission and fusion. Fission almost always involves the splitting of certain uranium isotopes (or other elements of similar size, such as plutonium). Fusion almost always involved deuterium or other forms of hydrogen fusing into helium.

Are there any other potential forms of nuclear power, even if they may be less efficient? Is there any form of nuclear power that involves intermediate-range elements (much heavier than hydrogen, but much lighter than uranium)? Are there known ways to extract nuclear power from any radioactive source, by using the radiation given off?