On sites using php, mod_gzip can save you bandwidth and is easy to implement. This Apache module compresses HTML on all newer browsers and older browsers that do not support gzip will work fine with uncompressed HTML. This has been posted on many blogs and forums, but I’ve noticed that many stock scripts do not have this simple line of code.

It’s easy to add to your site. Usually you just need to add it whichever file has the overall header for the site. (index.php. etc.)
After the <? on the next line add: ob_start("ob_gzhandler");

Click here to check if your site has mod_gzip enabled. Once you have it installed there is also a tool on that page that tells you how much bandwidth data you are saving per page view. More info here.