LessWrong, a group blog supposedly devoted to the science of living rationally, but in practice ground zero for some truly culty behavior. The Zizians arose from that community, for example.

LessWrong, an online community that started out as a place to discuss Yudkowski’s writings, who among other things is the If You Build It Everyone Dies guy.

LessWrong also directly gave us Effective Altruism and the Zizians.