Sef's ACR Film Grain
ACR Grain is the best way to create film grain that stays editable. It's a huge time saver to run it as an action as doing it by hand takes multiple steps. And if you know how useful film grain is in all sorts of applications from creative to technical, you'll be running this action multiple times a day.
Terms and Conditions
This action pops up a grain layer above whatever layer you have active in your photoshop document. It's applied to a layer filled with 50% gray set to Overlay blend mode. Try setting it to Soft Light for a lighter touch, or Hard Light for a strong look.
To edit the quality of the grain, double-click on the smart filter. Adobe Camera Raw will open, navigate to the Effects tab, and adjust Grain, Size, and Roughness to taste.
Note that the grain in your file will NOT update until you click ok in Adobe Camera Raw and return to your file. It's the downside of the ACR method, but the benefits of having editable grain outweigh the pain of going back and forth.