Wikibooks is a cool site that I just found out about from Zarco English. This is a collection of text books on any number of subjects that can be edited by the readers, just like a typical wiki. While this is a very intriguing site it does bring up the question how reliable is the data. However, there is a drop down button where people can rate the accuracy of content as well as other areas.
I highly recommend checking out Wikibooks by clicking here!!