This plugin will add a far future expiry header for various file types to improve page load speed of your site

Advanced Speed Increaser is a light weighted plugin to speed up wordpress website by using GZIP compression and setting header expiration for images.

Super Simple Speed is a stable and powerful plugin that dramatically increases your page speed without any hassle (or clutter).

This plugin will add expires headers for various types of resources of website and also help to add optimized minification and merging of resources to have better performance and speed optimization.