If you're lucky to have IIS6 serving your site, turning on the HTTP compression is a really easy way to decrease page loading times and cut bandwith. With compression enabled, web server can zip static and dynamic content before sending it over the wire. For this to happen the receiving side has to support compression too, as do all modern browsers.
There are two types of compression: static is essentially free. IIS will compress files on first request and store them on disk for future use, for minimal performance overhead. This works best for javascript and stylesheets. Dynamic compression allows .aspx output compressing; it's done on the fly and does add some CPU load. If processor isn't your bottleneck, enabling it is still a good idea.
For both types you need to define which file types should be compressed because the defaults are really lame: no css and no javascript.
On the new www.sitecore.net I've started with total frontpage load being 260kb and combining both compression types decreased it to 142kb. My favorite part is that prototype and scriptaculous (effects module only) javascript frameworks are 27kb total - at this point I refuse to take "prototype is too big" argument into consideration.
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