regen

This plugin compresses javascript files and CSS files loaded by the theme or other plugins automatically. 
Extra spaces, lines, and comments will be deleted. 
The compressor is based on <a href="http://code.google.com/p/jscsscomp/" rel="nofollow ugc">jscsscomp</a>.
<h4>Features</h4>
<ul>
<li>Auto-compression for the blog header.</li>
<li>Template tags which provide javascript compression.</li>
<li>You can turn on/off compressions in the admin page.</li>
<li>Editable CSS compression condition.</li>
<li>You can select where to output scripts.</li>
<li>You can exclude scripts from the compression if necessary.</li>
</ul>

Script Compressor

This plugin compresses javascript files and css files.

peterpfeiffer

Minify and Optimize your CSS by clicking one button. All you have to do is go to Settings and click CSS Minify. Activating the plugin there will minify and optimize your CSS. No settings required, just click one button.
So anything you can do to speed up the load time of your site should be worth investigating, right? One such thing is “minifying” the CSS files that are used in part to build your site. As is typically the case, there is a plugin available that can do the job for you – Suprise: It’s this one!
Code from this plugin is forked from Autoptimize a really great plugin. You can find it by searching for it here on wordpress.org in the plugin search.
<ol>
<li>
Upload the zip file and unzip it in the /wp-content/plugins/ directory.
</li>
<li>
Activate the plugin through the Plugins menu in WordPress
</li>
<li>
Go to Settings and then click CSS Minify and enable the options you want.
</li>
</ol>

CSS Minify

Way to go, peterpfeiffer! Your plugin contains the same files and functions as in Autoptimize and your plugin's PRO version provides what people could get for free in Autoptimize. Who knew "Copy, cut and paste" could be a business model?!

Autoptimize, but with most features moved to PRO

Minify and Optimize your CSS by clicking one button.

Script Compressor vs CSS Minify