Excellent sites for getting a good, healthy dose of theology:
“Man’s most valuable trait is a judicious sense of what not to believe.” – Euripides
I have included the links below simply to help people get an overview of what is considered either error (notice the lower-case “e”) or Error; that is, either movements or trends we need to be cautious about in the church, or heresy that posits as true Christianity; being cautious about the former because it can very quickly turn into the latter, as church history has illustratively shown to be the case.
It is vitally important to know where each of these branches or brands of Christendom are coming from so you will know how to sort through all the nonsense that is out there and come to a greater knowledge of Scripture itself, in order to love God and others to His glory. People are so non-forthcoming about their true underlying beliefs nowadays that it is crucial to have a theological grid in place to be as discerning as possible about what people are saying in their teaching.
Many false teachers and heretics have arisen (even within evangelicalism) during our day and we must be on the lookout, as watchful as hawks about what they are saying concerning the Scriptures. In addition, if you don’t understand where your opponents are coming from, how can you possibly know your own positions and how to defend those positions Scripturally and apologetically?
I have linked each of these straight off of Monergism.com’s site simply for the reason that there is no need for me to compile my own collection of bad theology when there is already one available that suffices. So start digging!