CSS Gzip | drupal.org

Based off of Javascript Aggregator this module will gzip your aggregated css files. For Drupal 6.x
Advantages

• It's compatible with Apache 1.3 (works great with cheap hosting).
• Compresses the content once and saves the result (less CPU load on server).
• Uses level 9 compression because it's only run once per file (smaller file size).
• In general, all you need to do is enable the module and it works (no core hacking/modifying .htaccess).

Disadvantages

• Requires Clean URL's in order to work (mod_rewrite).
• Tested on Apache, other http servers not tested.
• Requires download method to be public (check in admin/settings/file-system)

What about mod_deflate?

mod_deflate usually compresses the file for every request, the result is not cached. Thus choosing if mod_deflate is right for you depends on your apache server and how much CPU it has to spare. I believe that deflate is accepted by more browsers then gzip but i'm not 100% sure. The file size difference between the two compression methods is minimal. 99% of the time you want this module instead of mod_deflate. Discussion:#416808: I have mod_deflate - should I rather use CSS Gzip?
How do I know it's working?

The YSlow Firefox add-on has a tab called components; you can check for gzip compression there. See YSlow User Guide -> Components View.