Generations of children pulling out a World Book Encyclopedia or Encyclopaedia Britannica came to an end once this open-source online reference took off. Launched in 2001, Wikipedia now has 23 million articles and is the sixth most popular destination on the Internet. Traffic averages nearly 3 billion page views per month.
Encyclopaedia Britannica, meanwhile, put an end to its print publishing business this year, after sales of the 32 volume set (priced at $1,395) fell to just 10 percent of its 1990 peak levels.
Wikipedia's accuracy isn't always on target — something World Book and Britannica have pointed out regularly — but that hasn't stopped users from making Wiki the go-to spot for a quick fact check.