I just have to add a plug for an amazing resource, the Wikimedia Commons. I try to use open-source images on this blog, and this is where I get them. They’ve also been really valuable to us at the Exploratorium in terms of getting images that we don’t have to pay to use — it includes a lot of scientific images. Many images that are part of US government funded research projects are automatically part of the public domain. How great!
Wikimedia also includes, of course, Wikipedia, Wikinews, and Wikibooks. I’d be curious to hear people’s opinions on any of these, or other Wikimedia sites.