Anyone who works with the web likely knows that it can be a challenge to take a full web page screenshot. While the traditional go-to options for many web designers and developers is to print a page as a PDF or use the Mac standard screen shot keyboard shortcuts in OS X, for longer pages users have to scroll through a web page and take pictures repeatedly, then use a third party app like Pixelmator or Photoshop to join them together, a time consuming and tedious process. There’s a better way though, thanks to a free Mac app called Paparazzi, which offers a much easier approach to take complete screen shots of entire web pages regardless of how long or wide the website is.

Full article here:
Paparazzi Takes Full Web Page Screen Shots in Mac OS X Easily (OS X Daily)

NOTE: Depending on the browser used, it might be possible to get an extension or add-on that provides similar functionality from within the browser itself. For example, in Firefox you can use the ScreenGrab! (fix version) add-on to get this:

Screengrab! saves entire webpages as images. Just right-click on the page you want to grab and look in the “Screengrab” menu.
It will capture what you can see in the window, the entire page, just a selection, a particular frame… basically it saves webpages as images – either to a file, or to the clipboard, or upload to internet.