Federation of American Scientists is an anti-nuclear group whose website contains the texts of arms control treaties and other resources.
Our World in Data provides an easy-to-use collection of material such as charts and facts relating to nuclear weapons.
Next Generation Nuclear Network is associated with the Center for Strategic and International Studies, a US think tank. Its website includes a variety of information related to nuclear and other weapons of mass destruction.
Stockholm International Peace Research Institute provides a variety of WMD-related resources on its website, including short commentary pieces and other material.
Trinity Atomic Web Site includes a variety of photographs, PDFs of historical documents, and other resources related to the history of nuclear weapons.