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.

Matthew Haines-Young

Minification & concatenation of JS and CSS files can reduce the file size of your assets, and reduce the number of HTTP requests, helping improve page load times.
The plugin takes a manually defined list of script/style handles, and minifies and concatenates them into a single file which is then cached for future use.
Features.
<ul>
<li>Minifies & concatenates scripts and styles loaded using the WordPress dependency enqueueing system.</li>
<li>Handles scripts loaded in the header & footer.</li>
<li>Complete control – only a manually defined list of files is processed.</li>
<li>Multiple, independently processed files.</li>
<li>Minified files are only loaded if at least one component file should be loaded.</li>
<li>Compatable with localized scripts.</li>
<li>Helper tool – displays a list of scripts and styles enqueued on each page on the front end of the site.</li>
</ul>

MinQueue

Minify & Concatenate Enqueued Scripts & Styles.

WP CSS Merge vs MinQueue