How do you make images load lazily only when they are in the viewport?

Question

I am seeing a lot of sites these days, mainly tutorial sites that have a lot of images and they only load images further down the page once they come into the view port?

Answer 1

http://www.appelsiini.net/projects/lazyload
https://github.com/tuupola/jquery_lazyload

Demo:
http://www.appelsiini.net/projects/lazyload/enabled.html