I whole-heatedly embrace wikipedia. Especially the sources listed at the bottom. It's a great way to scratch the surface of a subject!
Wikipedia is a great source for plagiarists and therefore an easy way to detect them. It's far easier than googling random sentences or uploading to Turn It In. I fully embrace Wikipedia as a source for student essays and reports. I wish plagiarists would do so as well.
From the penalty (s)he received (when we report plagiarists, we have to specify preferred penalties for a first and second or later offense), I deduce that the student I caught plagiarizing from wikipedia last semester had done it at least once before. Some, apparently, are slow learners. I found the sources using one of the plagiarism detection services (which pick up wikip et al. very nicely), but if was well-honed professorial instinct that told me to check in the first place. Also, I'm always amused when students who plagiarized from something a bit more obscure insist that they never even looked at wikipedia. They didn't, but they also didn't realize how many other sites on the internet plagiarize from wikipedia (and/or each other). Mind you, I'm in the camp that thinks wikipedia is getting better and better (and, having written entries for the sort of "reliable" reference works which we often urge students to consult instead on a piecework basis, and seen the repeated warnings to my fellow-freelancers, most of whom were probably more economically desperate than I, not to consult wikipedia, am somewhat skeptical of such reference works as well. If nothing else, the level of pay compared to the amount of work required is a strong disincentive to really high-quality work, though I tried to keep mine half-decent, in part because my name was going to be on the entries).
"Wikipedia is the best thing ever. Anyone in the world can write anything they want about any subject. So you know you're getting the best possible information." Michael Scott, "The Office"
Ah, an authoritative source speaks of another.
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I teach my students to use it as a resource for narrowing down a topic, and as Alan from Apex points out above, the references at the bottom are often useful.But that's where the usefulness ends. I tell my students in no uncertain terms that a Wiki entry on their works cited page is a clear indicator of a lack of thought/effort, and that most professors, on seeing it, will judge the paper on it, whether they think it's fair or not. Better safe than sorry, I tell them--use it as a start to find other, better sources. Otherwise, you're just lazy.