NGINX, PHP-FPM, PHP-APC.
It's a powerhouse when it comes to serving static pages, so sites like Wordpress with cache's enabled etc, you can really do wonders with the speed. For real-time processing I haven't been able to justify the move as of yet (at least, not when we were testing the other month). Maybe some newer versions have come out?
We also looked into the packaged offer from http://cpnginx.com
. It's a great solution for a single site, or maybe a couple that you manage, but for shared hosting, since the .htaccess files all need to get rewritten, it can cause a cluster headache for anyone not familiar with NGNIX --- one of our major reasons not to launch on our regular shared servers.