"I don't have to cite this because it's in the public domain."
"If I found it on the Internet, it's in the public domain."
"This info is all over the Web. I can't even tell where it came from originally, so I don't have to cite it."
"I paraphrased the info, so I don't have to cite it. You just have to cite quotations."
All of the above are wrong, wrong...WRONG! Not only that, these common misconceptions can get you into a lot of trouble.
A common rule of thumb when you're writing a research paper: if you read, saw, or heard the information ANYWHERE, you need to cite your source. Unless it is strictly your opinion or your original idea ABOUT the information, you need to cite it.
So...even if you're referring to a bit of information that's on a thousand websites today, you should let your readers know which website you read it on. (Cite it!)