Greenshot - A Free Screenshot Tool Optimized for Productivity
Note: After creating the screenshot by dragging the cross-hairs across the image you want to grab, click "Open in Image Editor" from the list of choices that pop up immediately.
This tool meets my needs much better than the Snipping Tool (which only allows for hand-drawn shapes) or Google Docs (which relies on the Insert--Drawing functionality). In Google's defense, Google Slides does allow for the adding of shapes into a slide. I'm not sure why Google Docs doesn't.