Sandman
Any non-destructive fully reversible application. Photoshop, Lightroom, Aperture, iPhoto, DxO and many others.

nospam
photoshop can be non-destructive if the user uses it that way. it isn't normally.

Sandman
Depends on what you use in it. All layer effects and layer adjustments are fully reversible. I'd say that these days, most of your ordinary photo processing in Photoshop is reversible by default.

I.e. if you open a photo in Photoshop and click the Levels button in the palette, which is the easiest way to apply a levels adjustment, it's fully reversible. Only if you select Image ->Adjustments ->Levels do you get a levels adjustments that isn't reversible.

The filters menu is non-reversible unless you take steps beforehand to make them reversible (i.e. enable smart filters).

nospam
iphoto is not non-destructive. it makes a copy of an image when you change it and writes the changes to the copy.

Sandman
I.e. exactly like Lightroom. LR has a better UI for enabling and disabling effects, but the process is the same.

nospam
all you can do is revert to original.

Sandman
Or use the sliders in the other direction, same result. All adjustments can be reversed individually in iPhoto. Well, all except retouch and red-eye I think.

Also, the "revert to original" changes to "revert to previous" depending on what you're doing, so some edits can be reverted step by step.

It's not as sophisticated as LR or Aperture, of course, but it is 100% non-destructive.