When you're in the middle of a hot-and-heavy discussion, citing your sources can be quite helpful in supporting your opinion. When you cite a source, please include a link to the site so that others can read for themselves. However, also make sure that the site is one that's appropriate for a PG-13 forum.
Some sources have a bias, e.g. liberal or conservative, and that can radically affect how they approach a topic. Don't be surprised if the bias is evaluated and pointed out in the course of a discussion.
Wikipedia--this is a good starting source for a lot of discussions here, but don't use it as your be-all, end-all source--the authors of a given article can be just as biased as any other website. Since the articles can be altered and are not peer-reviewed, there may be mistakes or outright disinformation in some of the entries. The bibliography in each entry is valuable for the links to other sites, so check those out as potential sources.
Feel free to post other comments about sources and citations here.