Fireshot is a one of the few addons that are available for both Firefox and IE, it allows to take screen shots of web pages, edit them and annotate them in a simple editor.
There are some extensions like screengrab for taking screenshots, but this one also allows you to edit those screenshots. Using its inbuilt simple editor you can add text bubbles, highlight some objects in screenshots, add drawings to illustrate the image etc. It is easy to use and perfect for any one who uses screenshots to make a point. It is good for bloggers like me who use screenshots in posts.
Screenshots can be saved to your computer as PNG, GIF, JPEG or BMP images. There is an option to send them to their servers, or copy to clipboard or send it to an external image editor.
The addon is available to both IE and Firefox and the basic version is free to use, an advanced pro version is also available. The basic version is more than enough for most of the users, the advanced version will cost you $20.