Below is my current list of mechanisms of redshifting.

In addition to the Doppler effect we have:

1. Gravitational redshifting (See: Misner Thorne and Wheeler's "Gravitation")

2. Quantum fractional Hall impedance (Prof. R.M. Kiehn says for example:

"...the Action 1-form, A, has the physical dimensions of the flux quantum, h/e. The 2-form, G, has the physical dimensions of charge, e. The 3-form, A^G, has the physical dimensions of angular momentum, h, and the 3-form A^F, has the physical dimensions of spin multiplied by the Hall impedance, (h/e²) = h(h/e²) = h(Z_hall)."

He goes on to show that fractional Hall impedance is a built-in function of the vacuum which shall cause red shifting of photons.)

3. The spin field can cause both red and blue shifting.

4. The torsion field causes redshifting.

5. The subquantum aether can cause redshifting by several mechanisms.

For example, if you are at all familiar with Paul LaViolette's works, he shows a subquantum effect as being responsible for cosmological red-shifting. (There is evidence that there are subquantum entities. For example, the ZPE curvature is on the order of ~10^-66 cm^-2 [Wheeler].) Another related mechanism would be similar to LaViolette's description, having to do with the subquantum superfluid plenum creating a very small drag on photons over cosmic distances. The results of this kind of mechanism resemble the LaViolette mechanism, in that the same kinds of numbers are obtained by either method.

6 & 7. Whittaker and Bateman both showed redshifting mechanisms due to curvatures of space caused by both strong electric and magnetic fields, early in the last century. (Still looking for the exact paper on this topic, by Whittaker.) This is directly related to redshifting caused by gravity, because the ultimate causation of this kind of redshifting is the curvature of the space.

8. Topological redshifting, which can arise when a discontinuity is created in the vacuum by various topological means. Photons entering the discontinuity can lose energy in transit.

9. Bose-Einstein Condensates can redshift the frequency of light.

Actually, there are some additional mechanisms known from optics, but I can't recall those right now. (These effects are not well known.) I just remembered the edge of one of them! It has to do with effects on light propagating in a highly nonlinear, phase conjugate media. This can be found in Nature. In the vicinity of this is: Jack Feinberg's website

10. Quantum Scattering (See Redshifting)

There are other mechanisms available from studies of nanofabrications, but I would not expect these to be found in Nature.

So now I'm up to 11 different redshifting mechanisms, counting the Doppler effect, and I don't think this list has ended yet.