An experienced webmaster might have already tried it, a newbie might have just thought of it and a large number of people may have never had that idea.
Open-source, software that is used by everyone! Since a lot of php scripts such as wordpress, Joomla, Drupal and lots more are common nowadays, due to the fact they are open-source and free for use, templates and themes for them are developed everyday.
If you have used wordpress even only once, you’ve probably wanted to customize it a little and wanted a free template. Free templates and quality ones are available in hundred nowadays and after a little search you will find an appropriate that suits your needs.
After installing the template, you may probably notice that there are some links-which you are not allowed to remove- in the footer of every page. These links are known as sponsor links and are either links to the template designer’s website or peple who spent money buying that footer link.
And that’s exactly my point! have you ever thought of buying, or let’s say it better, “sponsoring” free templates and themes? You can spend 40 dollars one time fee and if it’s a professional and good theme, you’ll get hundreds of free backlinks when a lot of people use it.
Actually, you may even get good PR backlinks that way because there are lot of website of a sufficient PageRank(around 4) that use free templates for Joomla, Wordpress and others.
There are even designer that for around 70 dollars sell even the “designed by” link and you can add your URL there 
Wondering where you can find such templates/themes to sponsor? Visit the DigitalPoint forums and you’ll see lots of offers there! It certainly a worthy investment, you can try it 
Best Regards,
Thanos P.