I agree with Jill: giving credit and using quotation marks keeps it from being plagiarism.
Keeping quotes short also helps; "fair use" may even have a legal definition.
Rewording is enough to keep something from being plagiarism, but if the source really did have some unique insight or information, giving credit to the source seems like good manners.
I think that "courtesy to the author", even on the Internet, urges us to go beyond the letter of the law, even if it is a 99.9% unenforced law.
Perhaps, on the Internet, we should also respect a lesser standard: They put some work into creating content, and are entitled to the rewards of their effort. To copy the entire site or a whole article and NOT give credit would be gross theft, and potentially hurt the authors by giving away the authors' work without any benefit to them.
To use a short ATTRIBUTED quote may actually increase their traffic, helping them rather than harming them. I am guilty of that!
On the other hand, posting something on the Internet is a little like leaving a valuable object on the sidewalk, unguarded. Even if 99% of the people who walk past it abide by the honor system, when you come back a week later, someone is likely to have picked it up (or made a copy). That's not condoning plagiarism, but I wonder how copyright protection will continue to be enforced as more and more media becomes digital and hence easily copied? Even the music industry seems to be struggling with that.
