Wikipedia. Anyone with half a brain uses it. Anyone who has never heard of it really needs to see it. Wikipedia is an open source encyclopedia in wiki form, in which people are open to come in and edit or put in information on just about everything. Some people view this as a problem. They say if anyone can come in and edit info, then it's automatically wrong.
In school I have been told not to use Wikipedia. I asked why and they gave me the same un-provable reason: it's not correct. They FAIL!!! Wikipedia is the most trustworthy source you can use. If I can't use Wikipedia, then what do I use? Some old book from the last century? Losing confidence in that teacher, I go use Wikipedia anyway. Sure, people can go on and edit it, but if they do anything stupid, then Wikipedia blocks them, and Wikipedia has people that check everything out to make sure everything is ok.
So use it. Seriously