This breathtaking photo of a glacier in Iceland by Kenny Muir (flickr.com/photos/krmuir) is free to use under its Creative Commons license, provided you give attribution.

You’re familiar with the driver’s license, the fishing license, and the End Users License Agreement or GNU Public License you agree to when you install software. But if you’re not familiar with a Creative Commons license (sort of a happy medium between the copyright’s “all rights reserved” and completely public-domain “all rights released”), read on—especially if you’re in the market for free content to use in your film, podcast, blog, website, or any other project.

Creative Commons licenses let content creators keep their copyrights, but offer some conditional rights to the world to use what they’ve created. All Creative Commons licenses include attribution, meaning anyone using that content must give credit to the original author of the work. Then licensees can add conditions—say, allowing people to copy, distribute, display, and perform the work, but not to gain commercially (noncommercial), not to base other works on it (nonderivative), or, if derivative works are allowed, to require them to use an identical license (share alike). The “nonderivative” and “share alike” conditions contradict each other, of course, so a Creative Commons license can’t include both.

These CC licenses are popular with creative types who want their work more widely distributed. As the Internet circumvents the model of large corporate publishers controlling the distribution of works to maximize profits, the new idea is to build up an audience first and then leverage that momentum into bigger opportunities and projects—say, the indie band eventually lands a song in a TV commercial, or the blogger winds up signing a lucrative book deal.

Other CC-licensed content is free just because it “deserves” to be free—educational materials, for example. MIT OpenCourseWare (ocw.mit.edu), which includes lecture notes, exams, and videos from MIT’s curriculum, is published under a Creative Commons Attribution-Noncommercial-Share-Alike license. So you could post an entire MIT lecture on your own website, as long as you don’t pretend to have created it yourself or charge other people to access it. And if you improve on it somehow, you’ve got to release your new version under the same license. Same thing with United Nations University Online Learning (www.onlinelearning.unu.edu), University of NortreDame OpenCourseWare (ocw.nd.edu), Tufts OpenCourseWare (ocw.tufts.edu), and others.

Now that you know CC content is out there, where can you get your hands on some? Creative Commons has its own search engine at search.creativecommons.org, which also links to CC-licensed content searches on Google, Yahoo, Flickr, Blip.tv, Owl Music Search, and SpinXpress. (This same engine is built into Firefox’s search bar—click the tiny triangle on the left edge of the search box to see the list of engines you can search, and if Creative Commons isn’t in the drop-down list, select Manage Search Engines to add it.) Another large clearinghouse, Wikimedia Commons (commons.wikimedia.org), focuses on text and graphics.

We found a wealth of CC-licensed video clips on OpenSourceCinema.org, which can be downloaded, re-edited, and shared again. Need some music for your documentary or fresh loops for your GarageBand opus? Freesound.org is packed with CC-licensed audio samples, and iBeat.org boasts thousands of loops as high-quality WAV files. ccMixter.org has samples and remixes, including CC-licensed songs from David Byrne, the Beastie Boys, Le Tigre, Thievery Corporation, and more. If you podcast, bookmark PodsafeAudio.com and IndiePodcasting.com. And Flickr has more CC-licensed images than you can shake your mouse at, found with the Advanced Search.

Need more? Creative Commons has a giant list of such content directories (for audio, images, text, and even video) at wiki.creativecommons.org/Content_Curators. So go crazy, but follow the license stipulations, remember the generosity of the original creators, and always give credit where credit is due.