AdCiv has many links to Wikipedia articles pointing to further infromation on various topics.
Wikipedia is 'openly' compiled by a great number of contributors and editors and accuracy is not guaranteed. Like all encyclopaedias it is not a 'primary source' of information – as with any information it is worth referencing additional sources to be sure of the facts.
It is however a great example of what can be achieved modern collaborative internet-based 'open-source' mechanisms. It gains its strength from what is essentially a massive peer-review and low-bar of entry for adding new information. It tends to be especially good when dealing with more technical and scientific topics, although perhaps less good when dealing with more subjective topics, or very obscure topics where not many people may have reviewed the content.
It currently has well over three million articles in the English language alone (2010) and is around the eighth most visited website on the internet.