It is relatively easy to produce a professional-looking website on the Internet (including pictures, movies, and other "evidence") that presents an unusual or far-fetched argument such as Creationism (as opposed to evolution) or that global warming does not exist. Because such positions are held by only a minority of people, they do not get a lot of play on mainstream media. But the Internet, which spreads stories through "viral" propagation (people e-mailing their friends and so on), increases the reach of such fringe opinions.
The holders of fringe opinions are often strongly committed to their beliefs, and show a lot of zeal to convert others to their point of view. The Internet, based on the principle of free speech, does not shut anyone out, however unusual her/his claims might be. Our tendency to share unusual stories with our friends makes sure that many others are exposed to our "find". Finally, once we start believing in a certain theory, we tend to seek out evidence that agrees with it, and neglect evidence that goes against it (this is called the confirmation bias in psychology). All these factors make the Internet a fertile ground for the propagation of fringe opinions.
Faced with a large volume of information, that too well-organized and presented with seeming coherence, it is easy for us to mistake fringe opinions for scientific truth. However, most of us develop skepticism towards information with an unusual slant that comes from only one or a few sources, while most other websites on the same topic hold a different point of view. For instance, if a website tried to persuade us that the earth is flat (example) or that earth does not move (example), we could find literally hundreds of others that argue just the opposite; that should set us straight. Even if we did not find the opposing websites, a dose of critical thinking should serve us well - what is the basis of the claim, and how credible is the evidence presented.
A particular type of fringe opinion that flourishes on the Internet is conspiracy theories. From the Kennedy assassination to Princess Diana's death, from the claimed UFO sightings in Area 51 to the claim that the SARS virus escaped from a lab, conspiracy theories abound on the Internet. Some, like the theory that the US government staged the 9/11 attacks for political motives, receive so much attention that Government spokespersons have to explicitly deny them in the press. This is because the "evidence" for such conspiracies is skillfully presented: a mix of words, pictures and videos chosen to support the theory. Viral transmission on the Internet makes them common talking points among friends, raising their visibility and gaining more believers.
Conspiracy theories blur the distinction between fact and fiction and produce best-selling novels and TV shows such as The X-Files and Men In Black. One way to guard against taking them too seriously is to consider the intelligence, power and secrecy the government (or other conspirator) must be capable of to pull off the conspiracy. If the demands seem too great, we might be looking at a conspiracy theory.