Performance WordPress Plugins
Free
WP Super Cache
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.
Free
Picturefill.WP
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.
Freemium
W3 Total Cache
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.