Wikipedia is an excellent resource with around 3 million articles. Even if you take into account that some of the information may not always be correct due to any user being able to edit a page, on the whole most of the website is factual and it contains a summary of almost the whole depth and breadth of human knowledge. The GFDL license used also means you can use programs such as this plug-in to re-format and re-use articles from Wikipedia.

The Figure shows just the text of a Wikipedia article after extraction with this plug-in.

Using this plug-in you can extract just the text from a Wikipedia entry