Effect Stack is an OS X image editor that costs just $10 and weighs in at 3.9MB. It’ll process any image your throw at it, including RAW files, and its gimmick is that you can stack effects (hence the name) and shift the layers of this stack to switch things up.
It’s not bad.
To use the app, drag an image to the main pane and wait for it to load. Then start dragging effects from the list to the effects stack, and wait again.
I’m using an iMac from mid 2010, so this might be the bottleneck, but if you’re used to the instant nature of image processing on the iPad, even in apps using a similar effect-stacking metaphor, then you might be disappointed in Effect Stack.
The interface is basic but gets the job done. Effects are ordered alphabetically, which seems insane, and you can click on a little eye icon next to the name to preview the effect. A disclosure triangle opens the effect’s parameter adjustments, if any.
If you like the effect, of ahead and drag it up to the main stack, where you can switch them on and off and rearrange the stacking order.
I’d advice trying it out before buying. There’s a demo version which writes “Demo Version” on your image as you edit. I found that RAW images from my Fujifilm X100S took an age to load, and then even longer to render edits. JPGs are fine, as long as they’re small. Let me know if you have a more modern computer and have a smoother experience.