digitalapps

This plugins detects all of the styles queued via wp_enqueue_style. Contents of each css file are copied and stored in a single css file. This file is loaded on the frontend. The plugin wp_dequeue_style and wp_deregister_style all other css files. The result is your website only makes one single http request to load css styling. Bonus feature the contents of the css file are minified.
Simply flip the switch and the plugin will do the rest.
NOTE: I have only tested this plugin on my own sites running the AdSense Theme. If you encounter a problem with this plugin please let me know. I will be able to help you.

WP CSS Merge

This plugins detects all of the styles queued via wp_enqueue_style. Contents of each css file are copied and stored in a single css file.

neoslab

Powered Minifier allow you to reduce your page load and increase your performance by minifying your HTML source along with all the CSS and JS code present in your markup.
<h3>License</h3>
Good news, this plugin is free for everyone! Since it’s released under the GPL, you can use it free of charge on your personal or commercial site.

Powered Minifier

Honestly it's the best minifier plugin for Wordpress 

Simply Excellent

Reduce your page load by minifying your HTML source along with all the CSS and JS code present in your markup.

WP CSS Merge vs Powered Minifier