If you are on the web at all (and you wouldn't be reading this if you weren't), you've probably come across some kind of wiki. Know what a wiki is? Almost sounds like it has something to do with witchcraft but it doesn't.
A wiki is a web site that allows lots of different people to contribute to the content. Apparently the name comes from the Hawaiian "wiki wiki" which means quick. Probably the most well-known wiki can be found at www.wikipedia.org, the online "encyclopedia" of our times. You can learn quite a lot about the world at wikipedia because literally 1000s of folks have contributed articles and updates to articles on the site. They've got more than 2 million articles in English and a couple of million more in other languages.
Wikipedia, though, is not perfect. There are constant minor scandals about how bios of political candidates have been hacked and just today there was an article in the www.pennlive.com web site about how wikipedia falsely reported that the 2012 G8 summit would be held in Harrisburg, Pennsylvania. I would disagree however with the Harrisburg spokesman who said "You can't believe anything you see on Wikipedia".
There are some tremendous resources on history, geography, science, math and more to be found on Wikipedia. My recommendation is if you're quoting wikipedia on anything that could be remotely controversial, that you make sure you have other sources that also back up what you're saying.
Other wikis to check out include www.memoryarchive.org, en.wikibooks.org/wiki/Main_Page (open source books), www.wikia.com/wiki/Wikia (wikis on just about anything).