Despite their office productivity leanings, the iWork suite of apps (Pages, Keynote and Numbers) also serve as good image editors. With each app, you can crop and mask an image, as well as create alpha channels.
While a handful of useful paid-for screen capture apps are available for Mac OS X -- and many with very useful advanced features -- the one-two combination of the Screenshot Plus Dashboard widget and iWork can serve as an adequate pseudo screen capturing app.
While you can use Command-Shift-3 (or 4) as well as Control-Command-Shift-3 (or 4) to accomplish similar tasks, Screen Capture Plus also includes niceties, such as timed screen grabs and image file type options that, in addition to its straightforward functions, don't require much fiddling with.
After downloading and installing Screenshot Plus, make sure that it's configured to save files to the clipboard. As with many widgets, getting to Screenshot Plus's preferences requires a click on the little "i" on the bottom right-hand corner of the widget.
Saving your snaps to the clipboard will allow you to easily paste them into iWork to edit and annotate. Keynote is the iWork app used in the examples here, but these apply across all the iWork apps as well. And in the example, I've pasted a screenshot of iTunes with accompanying annotations noting album art, arrow links, and the Genius Sidebar.
While you can easily use Keynote's (or Pages and Numbers) tools to annotate your screen capture, I find that having them readily available on the side of your document better facilitates the workflow.
Using the "Shapes" tool, you can place bubbles and lines of all sorts onto your document. And to edit a shape's appearance, just click on it -- or highlight a group of shapes, if you want to do it en masse -- and click on "Inspector" (Command-Option-I). There, you can customize the shape's background, stroke, opacity, and much more.
After you've completed annotating and editing your screen capture, select all of its elements (minus the readily available template of shapes) and copy (Command-C) them.
Now, launch Preview.app, click on "File" and select "New from Clipboard" (Command-N). Your annotated and edited image capture is now ready for you to save. PDF, PNG, GIF and JPEG are a few of the many file types that you can use when you save your image.