W3 Total Cache
Plugin designed to cache and speed up your wordpress blog. With loads of setting you can minify css, cache pages, object, database, … You can even use Cloudflare for CDN.
Smush.it
A plugin for smashing (compressing) you pictures to the minimum size, but witouth loose of quality.
WP Socializer
Social networking integrated on every page and article of your blog. Lots of sites included.
GTmetrix
Speed analyzer of your blog with great suggestion on how to improve it.
Lazy load
Beautifully loading pictures when they are displayed in a visitors browser. Saves bandwith.
Loads of other plugins are available at wordpress and this is just a bit I use. Highly recommended.