Okay this is an amazing bit of code - If you have a website that's php based then you'll definately benefit from this. What it does is combining CSS files into 1 file, combining all javascript into 1 file, setting expiry headers, caching options and gzip options on the site so that there's less overhead, smaller filesizes and improved loading times. There are options for all these things.
PHP Speedy is a script that you can install on your web server to automatically speed up the download time of your web pages. PHP Speedy automatically joins together all the appropriate files and compressed them, greatly reducing the load time of the page.
from some blog I forgot to note last week (screenshot made of Yslow firefox plugin on one of my sites) More importantly, they have also made it available as a wordpress plugin!! Make sure you
download version 0.52 though because the homepage doesn't link to the latest version in a clear manner and version 0.47 doesn't work well on the latest wordpress. I'm informing the author.
Also in the options it offers to include prototype js libraries amongst others but this is unnessecary if you aren't using them at the moment and will increase your js size - so only tick them if you know you'll benefit.