I think that there is not any need to remark on the origin of shot peening, reported in almost every book dealing with this subject. Some of these remarks date back to the ancient Sumerians, who used to harden their swords by hammering the blades, getting great advantages with respect of their enemies.
In more recent times, something less than one hundred years ago, modern shot peening was on the way to industrial development as an effective treatment able to considerably increase the fatigue strength of mechanical parts. It is not important to recall that at that time, the reasons for such improvement were not really clear; indeed, this did not prevent shot peening from becoming more and more popular, also thanks to the research activities that were done and still are running.
And it is not by chance that one of the first sectors where shot peening was used is the automotive industry, probably due to the excellent combination of performance and production rates. That is to say that shot peening is not a “bottle-neck” of the production process, and this is a key factor for the success in mass-product manufacturing, such as for cars.
Nowadays, shot peening is no longer the only mechanical treatment that can be used for improving the fatigue life of machine parts and structural elements. Many other treatments exploit the same physical phenomenon of shot peening (the non-uniform plastic deformation induced by dynamic impacts), but different media and technology have been developed and are now potential candidates to be shot-peening substitutes.
Maybe the one that is most widely known is laser-peening, which substitutes the impact of the shots with a laser shock, with the final result of inducing a deeper compressive residual stress trend and a better quality of the surface finishing and roughness.
Another one is the so-called Surface Mechanical Attrition Treatment (SMAT), which uses a chamber where shots with a diameter larger than the ones used for shot peening are accelerated by a piezoelectric actuator and impact the target part with random angles, making the generation of grain refinement easier right down to nanoscale size and maintaining a better surface finishing. Grain refinement makes possible the achievement of superior properties and is at present, a very promising field of research in material science. 
Cavitation peening is another shotless allied process of shot peening, where the energy for inducing surface plastic deformation is taken by water cavitation by means of a dedicated device. And, again, we can cite waterjet peening in its different applicative ways, with pros and cons, and other related processes.
But, let me say, if we focus attention on the automotive field, then to my best knowledge, I have not read papers about the application of most of these treatments to car components.
Indeed, maybe this is due to confidentiality issues, but I think it is reasonable to assume that these treatments, in spite of their effectiveness, cannot replace shot peening in for the above field, since they are not able to guarantee the same production rates of "classical" shot peening. Some of them need a dedicated device, others require long preparation time or are not easy to perform with complex geometries, which are the cases with the largest notch effect (and where a mechanical treatment is more necessary).
At the same time, they are more and more attractive for particular and critical cases, where performance is a more important factor than productivity.
This is probably why these processes will keep on being shot peening-allied processes rather than competitors.

Shot Peening in the Automotive Industry
by Mario Guagliano
Contributing Editor MFN and
Full Professor of Technical University of Milan
20156 Milan, Italy
E-mail: mario@mfn.li