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.