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.
You can also have planets with powerful magnetic fields, a conductive body passing through them [generates electric currents](https://www.mtnet.info/division/papers/EMWKSHP_ReviewVolumes/1972Edinburgh/DyalParkin_1972EdinburghReview_PEPI_1973.pdf), that besides simply heating can also power chemical reactions directly (like charging up a chemical battery).

There is also chemical energy. If there is a constant supply or accumulated store of reactive chemicals, that can act as fuel or food or battery electrolytes. For example, there are [clouds of organic molecules in space](https://phys.org/news/2014-09-alcohol-clouds-space.html) that could rain down on to a planet and be used as 'food', or which could collide with other clouds and react. Or planets made of different chemicals could collide and then react with one another.

As rotating dust clouds collapse under gravity, they can form [accretion disks](https://www.universetoday.com/87597/zooming-in-on-proto-planetary-disks/) in which stars and planets form. The continual collisions generate heat, causing them to glow in the infrared. This likely includes cases too small to form a star (brown dwarf systems).

[Shock waves](https://en.wikipedia.org/wiki/Shock_waves_in_astrophysics) passing through interstellar gas clouds can produce intense heating. These shock waves are usually produced by stellar explosions (supernovas) but could in principle be produced by anything that causes high speeds, like the bow shock of a planet passing through a gas cloud.