Anytime you use free software, templates, or web code always be sure to check for hidden, illegally inserted spam back links. Lots of free content comes with this stuff inserted into some of the craziest and bizarre locations that are likely hidden from view. Anyone vaguely familiar with SEO knows that back links are important for search ranking but you should also know that spam marked backlinks hurt your page ranking.
Back links are a very good way to appreciate hard work whenever you use someone's elses free designs or codes. Just for the record, everyone should include a back link to the author of free software, free code snippets, images, designs, (or even help articles!!) as a display of respect and gratitude. This can easily be done on a single page or blog post with some good anchor text. However aside from moral obligations, you do not have to include back links. Anyone who forces or hides a back link in with free software is someone whose moral compass is a little off.
Slightly selfish authors of legitimate content may place their back links in plain sight. It is usually easy to spot in both the web browser view as well as the source code. If you an author and including back links into your developed content, please make the user aware and preferable to have a way for the user to disable it easily.
What you really need to look out for are hidden spam back links that are embedded into the source code somewhere. Hidden back links are often not allowed in certain software license's and are not allowed on Joomla Extension Directory. It is always best to look and see if your website is outputting a hidden back link because links to spam blacklisted websites will surely hurt your pagerank. To do this, I recommended downloading Firefox and downloading the extension FIrebug. In future releases of Firefox, Firebug will be integral.
Open Firefox, Open Firebug, then open a webpage of yours. On the Firebug menu, select Script (I prefer script over HTML for this task as its easier to read). You will notice your website after all PHP code is parsed and as it is sent to your browser for processing. Just slowly look through all of the code you see. Links are often specified like "<a href=". Just look through all of the code for links you do not recognize. You could also use find and search using terms <a, www, http://, stuff like that.
Once you find that your site has links embedded somewhere, the next step is to figure out where and how it is being injected. You will have to look through your Joomla template, overrides, modules, plugins. Sometimes the links are loaded under encoded scripting using base64encode. The hardest task is finding the culprit. You could disable new modules, plugins, and templates on at a time untill the problem disappears.