Get Autoptimize (compatible with WordPress 2.6 and above)
Note – I had troubles getting the “CSS optimization” portion of the plug-in to work. Activating it simply made this blog to vanish. That might be due to conflicts with some other plug-ins I’m using. The plug-in was released just two days back and there isn’t much help at the moment.
Update 2 : The plug-in’s author, Emilio, seems to be really enthusiastic. He has dished out a bunch of big fixing and new feature updates in the last couple of days. Autoptimize should now work for 99% of the cases. If you do run into problems, use the Support forum or drop him a reply ( @turl ) via Twitter.