An explanatory article in plain HTML embeddable with a simple php include() by other webmasters.
RSS feeds were still relatively new back then. Many CMS and weblogs offered it, but most users didn't know what it was good for. I wrote a relatively easy to understand article about it. About its history, its advantages, how to use it, which RSS readers were available, etc. It became one of my most frequently linked articles. Other website owners used it as a reference article next to their RSS buttons.
So I changed the format of this article and made it available in pure HTML, without CSS and without the <head> section, so that others could easily use it with a PHP include in one of their pages with their design. The article was included more than 100 times by others and generated a lot of backlinks to my domain.