If you're looking for a full-featured image editor at a relatively low price, Pixelmator fills the bill nicely. Sporting a smooth Mac-friendly interface, Pixelmator offers a layout much like Adobe Photoshop with separate toolbars offering standard selector and paint brush type tools, a color-picker, a layer manager, and a brush picker with large array of options, from "follow direction" to multiple jitter settings. Pixelmator also has support for Adobe Photoshop brushes.
Also like Photoshop, Pixelmator comes with all the image manipulation tools you might need for adjusting color, saturation, brightness and contrast, and many more. If you want to add effects to your images, you can pick from several different common filters like distortion, blur, sharpen, halftone, and stylize, along with more esoteric filters like clouds and noise. Essentially, Pixelmator gives you a lot of the same tools, effects, filters, and capabilities (like save for Web and trim) that you would find in Photoshop, though you won't find many of Photoshop's more advanced tools.
Perhaps the biggest difference most users with above average image editing needs will notice is Pixelmator's price ($59). It's important to note the demo version watermarks all your images, but once you see the amount of features this app has, you probably won't mind paying for all that functionality.