Wikipedia? Watch out!
I'd add to what eddo wrote that ALL students should always be VERY cautious in reading ANY Wikipedia article. As a former active contributor to the math/science articles (more than 10,000 edits from May 2005 through August 2006; see
http://en.wikipedia.org/wiki/User:Hillman/Archive for some indication of what I initially hoped to accomplish at Wikipedia--- I can't claim that the versions listed there are entirely error-free, just that I reviewed them closely and tried hard to keep errors out, to the degree this was consistent with respecting the views of other WP editors), I know all too well that:
1. some editors deliberately introduce errors, including hard to spot ones like sign errors (yes, I have specifically seen editors going around math articles doing precisely that), apparently because they enjoy antisocial activities,
2. some editors are very ill-informed or extremely bad writers, but don't hesitate to make large edits which largely destroy the value of previously good articles (one very common problem arises when an inexperienced writer inserts some new material without noticing that he has destroyed the flow of ideas or mangled a previously sensical segue into the next paragraph in the immediately preceding version--- I refer to phenomena like this which tend to gradually degrade articles over time, until a more knowledgeable and experienced editor takes the time to extensively rewrite and reorganize a long but disorganized and unreadable article, as "edit creep"),
3. some editors are on a mission to misleadingly portray (often as "anons" or under a "false flag") their own (obscure, inchoate, controversial or even downright cranky) "theory" as representing mainstream scientific opinion; while phenomena like "wikishilling" are often fairly easy to spot if you are experienced enough to be looking out for it, in some cases advanced knowledge of the subject in question might be needed to immediately tell when this is happening.
Wikipedia articles are also highly unstable because anyone can edit any article (well almost anyone can edit almost any article) at any time. If you feel you simply MUST cite a Wikipedia article, you should always use the "Permanent link" button (look at the left sidebar when you visit WP) to obtain a url pointing to the specific version which you read. But you should always consult printed sources and make a sustained and serious attempt to verify that what some version of a WP article claims seems consistent with what you read at independent sources. (Note that many "competing" on-line encyclopedias copy WP articles more or less verbatim en masse, as they are permitted to do by the GPL and related licenses, but it might not always be clear when this has happened because WP articles often change rapidly and drastically, particularly when they concern topics which have recently been in the news.)
On the other hand, some math related articles have been quite good and largely error-free for extended periods of time. Many experienced Wikipedians have observed that many "obscure" mathematical topics are well covered if one of the WikiProject Mathematics members happens to be a devoted fan, but unfortunately, one loon or troll can cause an utterly disproportionate amount of damage in a short time.
I feel there is a pressing need for a stable website offering advice to students, teachers, journalists, jurists, lawmakers, and others who extensively use Wikipedia as an information resource (scary? yes, scary!), since it seems clear that it has come to be generally regarded as too valuable not to use simply because it lacks any real concept of information integrity. I see grave social implications in the increasingly prevalent notion that "good enough information" means "plausible-appearing" (to a non-expert), rather than "authoritative", much less "true".
I actually attempted to write in my Wikipedia "user space" several essays describing my views on the promise and dangers posed by the Wikipedia for the citizenry of Earth, but Wikipedia has no provision for signed essays which cannot be vandalized by disgruntled editors holding a contrary view, and eventually I felt compelled to abandon my attempts to provide in Wikipedia itself an honest but searching examination of the challenges facing the Wikipedia in the so-called "year of quality" announced earlier in 2006 by cofounder Jimmy Wales. At some point I might take up this project again in another venue, however.
Chris Hillman