This is a very trivial question I suppose.
But, I dont remember any other sources of light mentioned anywhere other than the atoms emitting radiation due to electrons changing energy levels.
So, are there any other sources? What are they?
(Edit- In other words Iam asking for other light/electromagnetic wave production processes, other than the 'light emitted by electron energy level transitions')
There are many sources of light besides atomic transitions.
Objects with a temperature above absolute zero emit thermal radiation due to the random motion of their atoms and molecules. Instead of the bonds between electrons and the nucleus causing light emission, it is the bonds between atoms and molecules in the objects that creates light. The hotter the object, the more light it emits and the higher energy photons it emits (infrared for room temperature objects, visible light for much hotter objects like light bulbs and stars).
Radio and microwave antennas emit light by vibrating electrons at terahertz to gigahertz frequencies. Radio and microwaves are examples of non-visible light that is of much lower frequency than visible light.
Triboluminescence is where a crystal emits light when is is broken or scraped. Wint-O-Green Lifesaver candy is known to create visible sparks when chewed.
Sonoluminescence occurs when a bubble is trapped by ultrasonic sound waves in a liquid. As it oscillates in size due to the waves, it emits light during rapid contraction. The mechanism for this is unknown.
Cyclotron and synchrotron radiation occur when a high-speed charged particle (like an electron) is steered by a magnetic field. Accelerating charged particles causes them to emit radiation in a wide spectrum from infrared to visible to UV to X-rays.
Bremsstrahlung radiation is generated by suddenly slowing down a high-speed charged particle by slamming it into a dense material. The acceleration, much like the previous entry, causes the particle to release radiation, usually at X-ray energies.
Cerenkov radiation is emitted by charged particles that are traveling through a medium (like water) where the speed of light in that medium is slower than the particle's velocity. This is the luminous version of a sonic boom.
Any time charged particles interact, light can be emitted. This is because light is a wave of electromagnetic fields, and electromagnetism mediates the interactions of electrically charged particles. In all of the examples above, charged particles are ultimately the source of the light. In particle accelerators like the Large Hadron Collider (LHC), photons of light are by far the most common product of collisions between the accelerated protons and their underlying quarks. The collisions of charged particles causes energetic waves of electromagnetic radiation to be emitted, which we measure as photons.
An even more fundamental interaction is between particles and antiparticles. Since the antimatter counterparts of charged particles have opposite electrical charge, annihilating a particle with its antiparticle will often produce light, leaving no massive particles behind.
There are lots of other besides the ones I mentioned here: https://en.wikipedia.org/wiki/List_of_light_sources
Most of the visible light in the universe is Thermal Radiation. At high energies, there are also effects like Synchrotron Radiation or Pair Annihilation that don't require atoms to be present.
There seem to be confusion about the processes resulting in emitting radiation and sources of light - i.e., the objects where the light comes from.
The processes are any processes involving interaction between particles, coupling them to electromagnetic field. E.g., interactions between electrons and protons may result in emission of light - whether the electrons and protons are existing in a form of atoms, or molecules, or forming a crystal, collide in plasma or particle accelerator, etc.
Processes
Interactions within nuclei may also produce photons, but these are usually in the gamma range, i.e., these are usually not called light - which usually refers to the visible spectrum or nearby regions (like infrared and ultraviolet.) - see Electromagnetic spectrum and Do nuclei emit photons?.
Light sources: black bodies etc.
This light does not necessarily comes towards us directly - it may wander around and be re-emitted, reflected, scattered, etc. E.g., light in thermal equilibrium is referred to black-body radiation, and the objects emitting black body spectrum are often referred to as sources of light - in the sense that this is where we see the light coming from. However, the processes producing photons within these black bodies are the same that I mentioned above - mostly electromagnetic and nuclear interactions between particles.
See also:
How does radiation become black-body radiation?
Black body vs. Thermal radiation
Does fire emit black-body radiation?
How is light emitted by an incandescent lamp?
Remark: A mundane analogy is asking whether hot water comes from a teapot - yes, because it is heated in the pot, but no, because this is not where water originates.
Most of the photons in the universe arise in processes that don't involve atoms.
In the early universe most of the photons were produced by particle/anti-particle annihilation reactions. As the universe cooled to the extent that anti-particles were no longer produced, this locked in a photon to baryon ratio of about $10^9$. i.e. A billion photons for every proton or neutron.
As the universe cooled further, these photons are "processed". While scattering was common, emission and absorption processes such as bremsstralung (photons emitted by decelerating charged particles) and inverse bremsstrahlung (photons absorbed by accelerating charged particles) in a plasma kept the universe opaque to its own radiation. Since the radiation and matter were in thermal equilibrium, every absorption was statistically matched by emission, thus conserving the overall photon to baryon ratio.
After 400,000 years, the universe cooled to the extent that the plasma recombined to produce atoms and the universe became transparent to radiation. The atomic recombinations could themselves only produce 1 photon for every billion photons already present.
Those photons from the early universe are still present today in the form of the redshifted cosmic microwave background and still greatly outnumber any photons produced by atoms.
I am almost embarrassed to add this given the existing highly upvoted answers, but at least one important source of light has been omitted - the Cosmic Microwave Background is also technically light (photons).
This is remnant radiation from the early universe, when the initial plasma cooled down enough to allow the formation of atoms (mostly hydrogen), which were transparent to the light at the CMB frequency/wavelength (i.e. before this recombination epoch the CMB frequency photons were being absorbed by the early cosmological soup).
To explicitly answer the question in the title: the source of the CMB is not 'atoms radiating photons', but the 'origin of the Big Bang event', whatever that may be.
The answer depends on how you define light (visible only or any EM radiation), and whether you restrict the emitter to single atoms (or just nuclei, multiple atoms, molecules etc.). There are two really nice answers by @Profrob and @Markh, though I feel like there are three main topics that I need to add to because they are very important to understand:
https://arxiv.org/abs/2003.13952
Hawking radiation, there are many ways to explain Hawking radiation, but the bottom line is, the black hole's gravitational field's energy is transformed into the EM field's energy, that we perceive as quanta, that is photons.
Fission and fusion, these two processes can create photons in a non-trivial indirect way too.
So the answer to your question is that there are many non-trivial ways where the photon emission is not directly involving atomic systems, and sometimes, as a very surprising phenomenon photons can be emitted (transformed) by gravitational (black hole) systems' energy or even quasi-particles.
