The recent situation with the Yahoo! list that had used excerpts from at least one of my books without permission or attribution has sparked a number of discussions about when it is and isn't okay to republish someone else's work without specific authorization.
One person told me that he thought as long as he wasn't selling the material, it was okay. Well, if that's true, then that means I can break into your house, take your things, and as long as I give them away (rather than sell them), it's okay for me to steal from you.
Bottom line: it's not okay to steal. Period.
The only product writers have to sell is what they write. Yes, we often write things for marketing purposes that we put out into the public arena and encourage people to read and republish. That's how we get our name out there, and how we attract clients.
But the material that we write to sell is how we earn our living. When someone uses that material without compensating us, they are preventing us from potentially selling our work to their audience — and that has a negative financial impact on us.
It is gratifying to know that people think my work is worth sharing. But let's go for a restaurant analogy here: One of my favorite places is a neighborhood restaurant, Stefano's Italian Trattoria. I recommend it frequently. But when I say, "It's great, you should try it," I'm expecting the other person to go there and pay for a meal–I don't go into the restaurant, take food I don't pay for, and give it away. Of course, if I wanted to buy a meal for a friend, that would be fine–I could make my recommendation and Stefano's would be fairly compensated.
Here's a suggestion: If you find something on a website that you think others should see, don't copy the material, send them the link instead. That accomplishes your purpose of sharing the information and the author's purpose of driving people to their website.
There is a Fair Use provision to the copyright law that allows for brief excerpts of copyrighted material to be used in reviews and for educational purposes without going through the process of seeking permission from the copyright owner. If you're going to claim Fair Use, you should read the law and be sure you're following it.
In any case, when you use someone else's writing without attribution, at the very least, it's plagiarism; at worst, it's theft of intellectual property, and that's a crime.