Question: Are there other life giving sources of energy in space apart from stars? (like nebulae, radiation, etc.)? Or are all possible life giving potential sources some variation of a star?

Answer: 
There are three important sources of energy in planetary bodies (or their moons) that may be important in determining their suitability for life. All of these are evident in various bodies in the Solar System.

The first is tidal heating. When one object orbits another then there will be gradient in the gravitational force across their finite sizes. If this is combined with any orbital non-circularity or difference between rotational and orbital periods, along with some fluidity in the structure of one of the bodies, then it will be heated by frictional forces as the fluid moves in response to the changing gravitational gradient. This heating is most evident on Io in our own Solar System, the closest large moon of Jupiter, and is generally more important for objects orbiting close to more massive stars or planets.

The second is heating by radioactive decay. When planetary systems are born, they can incorporate significant amounts of radioactive material. Some of this has a short half-life and may be responsible for a short period of intense heating, but other isotopes are much longer lived. According to [this article](https://blogs.scientificamerican.com/observations/nuclear-fission-confirmed-as-source-of-more-than-half-of-earths-heat/), about 20 TW of power is generated inside the Earth from the radioactive decay of uranium, thorium and potassium. This sounds a lot, but is about a factor $10^4$ less power than we receive from the Sun.

The third source is just the "heat of formation". Gathering together the raw materials of a planet/moon and compressing it into a gravitational potential well, inevitably leads to the generation of considerable heat and it can take billions of years for this heat to leak out into space. Indeed, for the Earth, the heat of formation is probably comparable with radioactive decay in terms of supplying energy to the surface from the interior.
The answer by mentioned two internal sources of energy in planets and moons, etc.:

one) Tidal heating. In some situations tidal heating hasno significant effect, in ohters it can make a cold moon warm enough for life, in others it could cause a runaway greenhouse effect.

Two) Energy produced by the decay of radioactive isotropes.

But there is at least one other sourceo of internal energy in planets and moons, etc.

Three) The left over heat from the formation of the world as grains of dust, molecules of gas, and larger objects fell thousands or millions of miles toward each other and eventually impacted with great force.