Currently, I have downloaded Gimp on my Mac and using it to edit images. I don't mind spending money to buy other software if it makes things easier or faster. What is must have software for image editing on Mac?
It helps to think about what you mean by "editing." Do you need help with managing your photo collection, assisting with sending pictures to services, basic photo editing (how light/dark the photo is, managing overall color), or complex editing (multiple layers/channels, transparencies)?
The canonical list goes something like this:
iPhoto -- The modern version helps manage photos, send them to services, basic photo editing.
Aperture/Lightroom -- Better management of photos, more "professional" basic photo editing. It seems that some people prefer Lightroom over Apeture.
Photoshop/Gimp -- Not much help with photo management, but very good tools for both basic and advanced photo editing. I'm not that familiar with Gimp, so I can't get into that religious war.
I also know that there are web services that do basic photo editing, including canned effects as made popular by Instagram. I haven't used any of these services, so I can't comment on where they fit in the above list or how competent they are.
I use a combination of iPhoto (for managing the collection and quickly sending snapshot stuff to services like Facebook) and Photoshop for "serious" basic editing. I don't do a lot of advanced photo editing however, so I could probably get by with just Aperture or Lightroom.
I used Gimp on Ubuntu for years. Its a very powerful, if a bit strange, package. Great for the price.
Since I moved to a MacBook and OS-X, I use Aperture. Its available for about $80, and it does 99.44% of everything I want a package to do. I'm not a big fan of Aperture's "library" concept, where it manages all of your images, but many people swear that its great.
When I "edit" my photos, I don't do serious pixel magic. I crop, rotate, adjust colors, patch out blemishes, etc.
I also bought Lightroom, and for me, Aperture feels better. YMMV, of course. The recent versions of Lightroom are priced to compete with Aperture, maybe a bit more expensive, but still affordable for what they do.
I have never found that I need the serious stuff that Photoshop can do.
Try FX Photo Studio Pro - it is a very simple app for adding filters to images. It also has basic features, such as crop, rotate, and advanced features to adjust hue/saturation and stuff. You can check the trial version on CNET: http://cnet.co/KLslRB