If you need to take screenshots of Web pages--and annotate them and send them to others--you'll want to immediately download FireShot (free). This excellent Firefox add-in is the best tool I've yet seen for capturing and annotating Web pages.
When you want to capture a Web page, click the "S" icon on the upper-right of Firefox, and the page will open in FireShot. There, you can use a variety of annotation tools for marking it up, including drawing tools, shape tools, text tools, pointers, the ability to add graphics and more. When you're done, save it in a variety of common graphics formats: .jpg, .png, .gif., or.bmp.
FireShot has plenty of useful options and features, such as integration with your e-mail software to immediately e-mail a captured page. You can also capture not just the visible part of the page, but the entire page, if it scrolls down past your screen.
Given its tool set, simplicity, and that it's free, anyone who needs to capture and annotate Web pages will want to give FireShot a try.
Note: There is also a for-pay version of FireShot available called FireShot Pro, that costs $35. It has more powerful features, such as handling multiple captures simultaneously and unlimited levels of undo. When you download the free version of FireShot, you get the full features of Pro, but later reverts to the free version if you don't pay for it. There is also an Internet Explorer version of the program available as well.