One of the most useful things I've recently started using was i-frames, indent frames, I believe is its designation. They allow you to show content with preset width and height that changes frequently without having to go change all the pages in your site that display that same content, but just one. They're great for banner adds, polls, site/car/dog of the day/week/month, shoutbox's, menus, etc, etc, etc! Here's one:
I believe all fields are self-explanatory, so I'm just going to mention a couple of them that I find more important. First one is scrolling, which you can use to turn on/off the scrollbars, meaning you can use iframes as normal "includes", and visitors don't realize that they are separate elements. Second one, is the name attribute, which is specially useful to load content in the i-frame with a normal link, expliciting as target its name, like this:
<a href="URL" target="target_name">.