How to GZip Drupal 6.x's aggregated CSS and JS files
Why
I was tired of waiting for this to happen. It's simple stuff and the effect is pretty big if your content primarily consists of text. Well, it's not like this affects the text - that's already compressed by Drupal if you allow it to do so - what I mean is the compression ratio. If you serve megabytes of images, sound, and video, shrinking those few percent of JS and CSS won't have much of an effect.
However, if your site is more like this one you might be able to cut the size of the site in half. For people with a non-primed cache, that is. That's a really big plus if you get a sudden surge of visits. Nowadays the typical resource usage pattern seems to go from one spike to the next with rather little usage in-between. Take a look a Esoteric Curio's excellent article if you're interested in some more details.
As I previously pointed out even your regular visitors often won't have a primed cache, because the cache is simply too small for today's bloaty web. By the time they visit your site again the cache was already overwritten most of the time. Server-sided caching and compression to the rescue!