It's interesting, because the article also talks about the licenses:
Digging through ÔÇ£freeÔÇØ sites to sort the ÔÇ£free beerÔÇØ from the ÔÇ£free speechÔÇØ content is quite a chore. Many of the sites are not useful for free culture projects, and many make it very difficult to tell. Fortunately for you, I took notes! Here you will find 8 sites with free-licensed content, 8 more with licenses that youÔÇÖll probably find acceptable for many projects, and 20 others that might be useful on some projects if youÔÇÖre not a purist. There are also 22 sites I have to warn you away from, because their terms are incompatible with use in free-licensed productions.