(Edit: Yes, I realize this isn't new or controversial. I wrote it for my own reference.)
No heading here, just watch out for those slings and arrows.
Wikipedia. Even linking it without making a point somehow seems criminal. After 2001 anyone who listened when the teacher explained about a bibliography and citing sources probably remembers wikipedia is not a valid source. That same article does make the point that wikipedia often has decent articles which themselves cite sources. I'll get to that.
The problem is that people can edit a page, put up information, and then have it widely available on the internet, with little fact checking. The information may take some time to appear, but will also take time to disappear. I'm sure many people have friends or heard of people who deliberately post wrong information on Wikipedia.
As a source, it is unreliable. Consider Wikipedia's changing nature; if you use a wiki page as a source, you cannot be sure it will say the same thing tomorrow. Maybe you agree with it now, but in a week, the wording is changed and the change is more technically correct, but you no longer like it. Letting other people control the point you are trying to make gives them power over your article, book, or paper. So, just to be sure, what is a valid source? Good question.
Don't let me push you too far away from Wikipedia. If you are not engaged in any highly technical writing (academic, or published works,) then you probably do not need to worry about your sources quite as much.
Blogs are a great example; if you use poor sources, you are only accountable to yourself and your readers (who are honoring you by reading your blog anyway.) Not to say you should post poor or wrong information, simply that nearly making your point might be just as good as making it exactly.
Giving readers something contentious to comment about might build an audience. If that matters.
User editing, the very thing which makes Wikipedia a poor source, can be a plus in the right situation. Consider this real scenario: A friend of mine has a specific type of intense headache. I had never heard of these headaches, so I looked them up and, of course, a wikipedia came up first.
I read the wikipedia page, saw when it was edited, examined the sources, and skimmed most of those, only to see them mostly agree with the Wikipedia article. I then searched the web a little more and found a few pages that disagreed with the article, but they were published a few years ago.
I check the newer sources, listed on Wikipedia, and see they have directly refused the older sources. In this case, incorrect information was taken down as part of the user's interaction.
In the middle of my story, a single important fact might have be missed. When I read the Wikipedia page, I looked to see if they had sources and then read (or at least skimmed) any they listed.
Consider the guidelines for decent sources (as listed above, but re-linked here,) while checking these pages too. A person could create a false source just as easily as posting incorrect information on Wikipedia.