A slightly controversial subject today with regards to students... Back in the summer I presented a paper which argued that Wikipedia could, within limits, be a better source of information than its reputation suggests - most lecturers are reluctant to accept student work which cites Wikipedia for a number of reasons, the main one being that the site can be edited by anyone and so is open to abuse. There is good material to be found on Wikipedia, but separating the high quality from the poor is not always easy, and students would usually be advised to use it only as a starting point for learning about a topic.
To assist with using the site sensibly, you can now access an 8 page booklet entitled Evaluating Wikipedia Article Quality. This contains simple, easy-to-follow advice on how to assess whether pages on the site are likely to be accurate or well-researched, and teaches you how to follow the development of an article to see how it has evolved. While it's not definitive, it should help you to start considering some of the critical judgments you need to make when using Wikipedia, so that if you are going to visit the site, you can at least be more confident in choosing what material may be useful, and what pages should be avoided.
To view the booklet, click here.
(Image taken from http://xkcd.com/903/ under Creative Commons terms - click on image for a larger version)