The Page of Fu Manchu: a rarely-updated site that nevertheless provides a good starting-point for those who want an introduction to the Devil Doctor, and will include periodic updates on publication news.
Dr. Hermes Reviews Fu Manchu: as the name implies, these are detailed reviews of the Sax Rohmer and Cay Van Ash books, which include spoilers (which are pointed out, for those who do not want to know). The site also provides other insights into the novels and perspectives on the times in which these works were written (to explain, if not excuse, some of the racially offensive passages, for example).
The Page of Sumuru: devoted to Sax Rohmer’s other great fictional villain, the female counterpart to Fu Manchu. These novels are currently available only in Canada in an omnibus volume.
The Fantomas Website: the French villain gets his own site – a remarkably complete site that covers the villain himself, his creators, the books and the films based on the character (note: see Black Coat Pressfor continuation novels by other authors).
Madame Atomos: of course she has her own site! – she’d release giant scorpions on you if she weren’t so honored. This site discusses the books and includes a timeline of the villain’s life and career. It also notes the French comics that featured Madame Atomos as well.
Poor Dr. Mabuse, I can find no website devoted to him.