Awesome Screenshot is a neat and very useful Safari Extension, which lets you capture, annotate and upload screenshots to awesomescreenshot.com or save them to your hard drive.
Clicking the extension icon in the Safari bar reveals a tastefully designed toolbar. You can annotate your screenshot by drawing freely or use boxes, circle or lines. On top of that there is a tool that blurs sensitive information that you would like to keep hidden. Once you are done you can upload the image to awesomescreenshot.com, save it onto your hard drive, or drag and drop into the email window if you use Apple Mail or any other app that supports drag-n-drop.