Em Shreve

This plugin implements gzip output compression to speed up load times.
The plugin also does a check to see if browsers are incorrectly saying that they do not support GZIP when they actually do. This can speed up load times for a significant portion of users.
<pre><code>For requests with missing or mangled Accept-Encoding headers, inspect the User-Agent to identify browsers that should understand gzip.
* Test their ability to decompress gzip.
* If successful, send them gzipped content!
</code></pre>
See:
<ul>
<li>http://www.stevesouders.com/blog/2010/07/12/velocity-forcing-gzip-compression/</li>
<li>http://en.oreilly.com/velocity2010/public/schedule/detail/14334</li>
<li>http://wpdevel.wordpress.com/2010/07/13/forcing-gzip-httpwww-stevesouders-co/</li>
</ul>

Force gzip

Implements gzip output compression to speed up load times and does a check to see if browsers are incorrectly saying that they do not support GZIP whe …

riotweb

About
This plugin will instantly speed up your WordPress website when the plugin is activated. 
Gzip is the most popular and effective compression method currently available and generally reduces the response size by about 70%!
Important 
This function changes your .htaccess file. In some cases this might result in a 500 error. If you’re not sure whether you can run Gzip ask your hosting company. 
The only way to fix this is to be able to edit the .htaccess file from outside WordPress (via FTP).
Gzip Checker 
NEW – You can now check if Gzip is enabled for your website. Go to “settings” and select the “Instant Gzip Compression” tab.

Instant Gzip Compression

Speed up your Wordpress website with this Gzip compression plugin.

Force gzip vs Instant Gzip Compression