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: 
ProfRob makes a good point about tidal heating, but you can have internal heat sufficient for life even without this. The heat I'm referring to is heat trapped in a planet's core from its formation, which slowly and reliably leaks out through geothermal vents.

We observe this at the bottoms of Earth's oceans, where thriving colonies of deep sea organisms live in total darkness, feeding off the heat and chemicals that come from these vents. No sunlight supports these organisms, only geothermal energy.

Note that how long this is useful depends on your planet's size. Mars used to have a liquid core, but has since cooled down enough to have its core freeze since it's smaller and therefore cools faster. The larger your planet, the longer it'd be able to provide this trickle of heat.

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Edit: it has been made aware to me that Earth's internal heat is mostly driven by radioactive decay nowadays, which I didn't know. The point still stands however that a planet's original heat could be sufficient to jumpstart life. I'm not sure how long it could provide for this before something else would need to kick in though.
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.