Quick rundown: If you wish to make a link for a user to start a new topic, you can automatically fill the subject field with text of your choice upon the loading of the page!
In a bit more detail: For example, suppose you run a site where users post their review on a product you sell. The user posts their review on your forum, and names it "Review on Product X". The post is then syndicated to another page on your site. On this page, you include a link where users can reply to another users reviews in the forum (such as posting.php?mode=reply&t=1). With this MOD, you can make it so the subject box is automatically filled with the subject name of your choice (it can always be changed by the user). So rather, than linking to posting.php?mode=reply&t=1, you can now link to posting.php?mode=reply&t=7&subject=Re: Review on Product X and the subject field for the reply would automatically have Re: Review on Product X in it. It can also be done for new topics, by adding the &subject= on to the end.
Download this MOD from the phpBB.com MOD Database