user9943: I am running into the same issue time and again. It may be too broad of a question, and it may be heavily opinion based, but I still wish to bring it up.
First off, I am trying to use articles from credible journals for my answers. However, it does happen that I simply can't find the proper material in the literature. For example, specific information on anatomical features, the side effects of drugs, or the specific symptoms of diseases are notoriously difficult to dig up without the availability of appropriate textbook or reference works. Or, to provide an example for all the Cell Biologists and Geneticists here, imagine you have to dig up how to prepare all the various solutions needed for a home-brewn fenol DNA extraction without Maniatis at hand. Would you sift through articles to find the one that happens to include the composition of all the buffer solutions and organic solvents?
Personally, I am hesitant using wikipedia as a source for the basis of my answer, as it may potentially list unfounded and unreferenced stuff, and sometimes even plain fallacies*. This aside from the fact that many wiki pages are either too elaborate (popular ones such as 'Neuron') or very short and unintelligible (unpopular ones such as 'Bipolar neuron'). lasrt but not least, it is kind of a dynamic website, so info may become deleted or changed. So the many comments placed under questions with a wiki page linked and the message 'Have you done any research?!' are not too useful in my opinion, but that aside.  
There are other sources from which I simply do not know the credibility, but that do pop up a lot in my searches and seem credible but I simply can't assess whether they are worthy sources. I will add a list hereafter of typical examples. I prefer not to use them, or only sparingly and reluctantly in my answers**. I would, however, love to use them freely given the amount of information they contain. Moreover, I have noticed that other people use them quite frequently as a basis to back up their answer (e.g., Mayo Clinic is often used). If folks could shine their light on the value of these sources that would be really helpful: 

Wikipedia.org {Probably/arguably the most referenced source at Biology here. Personally, I avoid using it, but I have put it into the list for discussion}.
Encyclopedia Britannica.com {wikipedia-like, but not open-access, i.e., not everyone can add stuff. It's a paid site when one wants full access}
Drugs.com {encyclopedia-like site on pharmaca}
Mayo Clinic.org {a site from a nationwide US clinic with a lot of accessible and interesting information, but directed to patients, not health care providers}
WebMD.com {a site with descriptions of diseases and pathologies}

I might be adding a few more sites, but these are the most important ones, or at least give a fair idea of the type of sites I am referring to. 
*However, I do use wikipedia to link specific words to explain jargon. 
**University websites I use without much hesitation. For example, web pages of labs associated with credible universities.

user6307: I believe I try to put in some kind of disclaimer in the cases where I can't or won't find a proper reference. Even something as simple as, for example, "according to wikipedia" instead of a matter of fact statement. 

user4101: Mayo and WebMD are fine, in my opinion.  They are widely known, universally appreciated, and are essentially the web-standard.  They should of course only be used for information and definitions, as we don't give medical advice here.
I don't like drugs.com, mainly because it got shit on in a recent US Supreme Court case, but it is I believe of high-enough caliber.  At the very least it's pretty well trusted which is what matters.
I am perhaps biased, but if given a choice between Encyclopedia Britannica and Wikipedia, I will choose Wikipedia.  The correctness of the two is largely the same (some studies go either way) but I think it's more useful for a number of important reasons:

It's recognizable.  Everyone has been there so it's not a new site to navigate, figure out where information is, etc.  Users will likely be expecting it.
It's content driven, so while the site has bloated over the years, it is still a simple article on a page.  EB splits content onto other pages (drives up page views), displays user comments, etc.
It is super, super easy to find more information on related topics, and not just other links; the references and citations are the best part of a Wikipedia article.

All this with the caveat that none of these are primary sources, and most aren't even secondary.  You can never beat out primary or even secondary sources.  I'll link to genecards before enWiki.
Wikipedia is probably most beneficial for some of the amazing artwork and images, which can be super helpful in answers to questions here.  That's not to be overlooked.

9943: <p>I am running into the same issue time and again. It may be too broad of a question, and it may be heavily opinion based, but I still wish to bring it up.</p>

<p>First off, I am trying to use articles from credible journals for my answers. However, it does happen that I simply can't find the proper material in the literature. For example, specific information on anatomical features, the side effects of drugs, or the specific symptoms of diseases are notoriously difficult to dig up without the availability of appropriate textbook or reference works. Or, to provide an example for all the Cell Biologists and Geneticists here, imagine you have to dig up how to prepare all the various solutions needed for a home-brewn fenol DNA extraction without Maniatis at hand. Would you sift through articles to find the one that happens to include the composition of all the buffer solutions and organic solvents?</p>

<p>Personally, I am hesitant using wikipedia as a source for the basis of my answer, as it may potentially list unfounded and unreferenced stuff, and sometimes even plain fallacies<sup>*</sup>. This aside from the fact that many wiki pages are either too elaborate (popular ones such as '<a href="https://en.wikipedia.org/wiki/Neuron" rel="nofollow">Neuron</a>') or very short and unintelligible (unpopular ones such as '<a href="https://en.wikipedia.org/wiki/Bipolar_neuron" rel="nofollow">Bipolar neuron</a>'). lasrt but not least, it is kind of a dynamic website, so info may become deleted or changed. So the many comments placed under questions with a wiki page linked and the message '<strong>Have you done <em>any</em> research?!</strong>' are not too useful in my opinion, but that aside.  </p>

<p>There are other sources from which I simply do not know the credibility, but that do pop up a lot in my searches and <em>seem credible</em> but I simply can't assess whether they are worthy sources. I will add a list hereafter of typical examples. I prefer <em>not</em> to use them, or only sparingly and reluctantly in my answers<sup>**</sup>. I would, however, love to use them freely given the amount of information they contain. Moreover, I have noticed that other people use them quite frequently as a basis to back up their answer (e.g., Mayo Clinic is often used). If folks could shine their light on the value of these sources that would be really helpful: </p>

<ul>
<li><a href="https://en.wikipedia.org/wiki/Main_Page" rel="nofollow"><strong>Wikipedia</strong>.org</a> {Probably/arguably the most referenced source at Biology here. Personally, I avoid using it, but I have put it into the list for discussion}.</li>
<li><a href="http://www.britannica.com/" rel="nofollow"><strong>Encyclopedia Britannica</strong>.com</a> {wikipedia-like, but not open-access, i.e., not everyone can add stuff. It's a paid site when one wants full access}</li>
<li><a href="http://www.drugs.com/" rel="nofollow"><strong>Drugs</strong>.com</a> {encyclopedia-like site on pharmaca}</li>
<li><a href="http://www.mayoclinic.org/" rel="nofollow"><strong>Mayo Clinic</strong>.org</a> {a site from a nationwide US clinic with a lot of accessible and interesting information, but directed to patients, not health care providers}</li>
<li><a href="http://www.webmd.com/" rel="nofollow"><strong>WebMD</strong>.com</a> {a site with descriptions of diseases and pathologies}</li>
</ul>

<p><sup>I might be adding a few more sites, but these are the most important ones, or at least give a fair idea of the <em>type</em> of sites I am referring to</sup>. </p>

<p><sup>*However, I do use wikipedia to link specific words to explain jargon.</sup> </p>

<p><sup>**University websites I use without much hesitation. For example, web pages of labs associated with credible universities.</sup></p>
 6307: <p>I believe I try to put in some kind of disclaimer in the cases where I can't or won't find a proper reference. Even something as simple as, for example, "according to wikipedia" instead of a matter of fact statement. </p>
 4101: <p><strong>Mayo</strong> and <strong>WebMD</strong> are fine, in my opinion.  They are widely known, universally appreciated, and are essentially the web-standard.  They should of course only be used for information and definitions, as we don't give medical advice here.</p>

<p>I don't like <strong>drugs.com</strong>, mainly because it got shit on in a recent US Supreme Court case, but it is I believe of high-enough caliber.  At the very least it's pretty well trusted which is what matters.</p>

<p>I am perhaps biased, but if given a choice between <strong>Encyclopedia Britannica</strong> and <strong>Wikipedia</strong>, I will choose Wikipedia.  The correctness of the two is largely the same (some studies go either way) but I think it's more useful for a number of important reasons:</p>

<ul>
<li>It's recognizable.  <em>Everyone</em> has been there so it's not a new site to navigate, figure out where information is, etc.  Users will likely be expecting it.</li>
<li>It's content driven, so while the site has bloated over the years, it is still a simple article on a page.  EB splits content onto other pages (drives up page views), displays user comments, etc.</li>
<li>It is super, super easy to find more information on related topics, and not just other links; the references and citations are the best part of a Wikipedia article.</li>
</ul>

<p>All this with the caveat that <strong>none</strong> of these are primary sources, and most aren't even secondary.  You can never beat out primary or even secondary sources.  I'll link to genecards before enWiki.</p>

<p>Wikipedia is probably most beneficial for some of the amazing artwork and images, which can be super helpful in answers to questions here.  That's not to be overlooked.</p>
