Few things are as annoying as building something that works perfectly when you create it, but fails when you take it out of the lab. That's how site owners can often feel when content editors create piles and piles of Drupal nodes full of relative URLs in images and links. They look fine on the site, but if the content is syndicated via RSS or Atom, sent out in an email, or otherwise repurposed in another location, the links break. Even worse, hand-made links and images entered while the site is under development can easily point to the outdated "beta" URL. Who can save the day? Pathologic module, that's who.
Pathologic is an input filter -- to install it, you drop the module into your Drupal site and add it to one of your text formats -- Full HTML and Filtered HTML, for example. Whenever content is posted in a format configured to use Pathologic, it will scan the content for URLs and tidy them up. Relative URLs like /node/1 get turned into absolute ones like http://example.com/node/1, URLs pointing to alternative versions of your site like dev.example.com are replaced with your public URL, and so on.
Pathologic can also standardize the protocol of links inside your site's content. If users edit content over a secure connection, for example, it's easy to mix links using the http:// and https:// protocols -- something that can lead to annoying warnings on some users' machines. For developers with exacting URL-correcting needs, it also supports custom URL modification hooks. Using those hooks, your site's custom fixes (replacing MP3 links with a URL on a different server, for example) can piggyback on Pathologic's configuration and logic.
Pathologic is an efficient workhorse of a module that solves an annoying problem efficiently. If you've run into problems with relative links and staging-server URLs breaking links and images on your RSS feeds, you owe it to yourself to check it out!