Depends on the type of editing you're talking about. From what I've seen, you've got a couple options:
Organizing photos: iPhoto, Aperture, or Lightroom. These have limited editing features and may be fine for you. iPhoto comes on the computer, and trial versions of Aperture and Lightroom are available so you can see if they would meet your needs.
Editing photos:
Photoshop Elements, for fairly simple photo editing that goes just beyond iPhoto et al.
Pixelmator, a lightweight equivalent to Photoshop. Many people find that for $29, it does everything they need, even though it does not contain all of the features of Photoshop.
Photoshop, the industry standard. Depending on your needs, this may or may not be necessary. Certainly costs the most, but it does provide the most features as well.
jW