Lightbox
Lightbox is a very useful JavaScript implementation that allows you to overlay images on the current page in Web 2.0 style. Implementing it is easy: you simply include three JavaScript files and one CSS file in the <head></head> section of the pages you want to use it on. After that, add a rel="lightbox" attribute to the links you want to use Lightbox on, and you’re done!
Initially I was worried about the size of these included files together, being around 70 kB. But if you deliver your JavaScript and CSS files gzipped to the requesting browsers, the total file size is only 17 kB. A number I find quite acceptable, especially considering the cool effects you get in return.
