Minify is a PHP library that attempts to fix this problem by combining multiple CSS or JavaScript files into one download. By default, it also removes comments and unnecessary whitespace to decrease the amount of data that must be sent to the browser. Most importantly, it does all of this on the fly and requires only a few simple changes to your existing web pages.

He gives a before and after example of the things it can do - mainly combining multiple tags (like link and script) and their information into one. Check out its page on the Google Code site for more information.