Automatically compress and optimize images that are uploaded to WordPress on your own server. No need to use external third party services, and easy to customize.
Get a running list of MySQL queries on each page using a link in your admin bar. Groups queries together by different parameters and troubleshoot performance issues and errors.
Cache pages using mod_rewrite or PHP automatically. There are lots of ways to customize the plugin, but the main appeal of this plugin is that static pages are created on the fly without having to set too much up.
Picturefill.WP provides an easy way to add responsive images to your WordPress site, using the Picturefill polyfill developed by Scott Jehl and automatic image sizing.
W3 Total Cache is a "performance framework" that allows you to enable and tweak Page, Database and Object caches, utilize Browser compression, integrate with your CDN and plenty more.