Last week I participated in a Slashdot discussion on copyrights. The discussion was specifically about the Creative Commons initiative. For those who don't know, Creative Commons (CC) provides free license texts that you can attach to copyrighted content that license the content for use by others in specific situations and with specific conditions. A typical CC license would allow people to use content for non-commercial purposes, for example. People confuse CC with copyrights, but CC is about licensing copyrighted material, which may not be as simple as you might think and is why CC was developed. Many blogs (but not this one) are licensed under a CC license.
The reason I bring this up is to point out that there are obstacles to using other people's content. In almost every country in the world today, including the United States, content is granted copyright protection automatically, as soon as it's created. That protection exists whether or not there's a copyright notice. In other words, a lack of notice is not an invitation to copy.
If you want to use someone else's content, you must get permission from them. This is where licenses like those from Creative Commons come into play. Many times, the copyright owner (note that this may not be the actual person who created the work, depending on different factors) gives explicit permission to use the content in certain ways. For example, I've licensed my recent articles The AdSense Formula and When AdSense Makes No Sense to any site or newsletter that wants to reproduce them, providing that they are reproduced in full with no changes and that the copyright message and the biographical information that accompany them are also left untouched. The result is that you can now find those articles on many different sites like SelfSEO or EZineArticles. This provides me with a bit of extra publicity for my AdSense book for non-techies Make Easy Money with Google.
Remember, though, that once your content is on the Web it's easy for others to copy it. For example, my original Google AdSense Tips keep finding their way on other people's sites even though I haven't given them permission to do so. Someone had even copied them to a site and placed a Creative Commons-type license on them, which meant that other sites picked them up and republished them, thinking they were free. If it's not yours, be wary of using it, and check out its pedigree as much as you can. (I discuss this all in Chapter 3 of the book, by the way.)