These sites have general information on German film history and/or Weimar cinema in particular. Some of them are in their native German, but you should be able to navigate them if you have a basic reading knowledge of the language.
A Hamburg-based "lexicon" of German film history and the German film industry. The biographies page has a comprehensive listing of directors, writers, cinematographers, and actors. CineGraph is a professional organization and much of its material will prove useful to your research. In German, with a few passages in English.
Stichworte zur Geschichte des deutschen Films
Prepared by the Goethe Institute, this page gives a brief but helpful history of filmmaking in Germany from 1895 to the present. Key terms like UFA, Expressionism, and Kammerspiel are defined in well-crafted paragraphs. In German, but the summaries are comprehensible if you have had any training in the language whatsoever.
German Film Home Page
A very general introduction to German cinema, written by Dr. Sandy Frieden of the University of Houston. This site is intended for student use, and its most important section is a bibliography of film theory and criticism relating to Germany.
A Source Book for Early German Film
The creators acknowledge that their site focuses on The Cabinet of Dr. Caligari and the early career of Fritz Lang, but there is also information on other major Weimar films and filmmakers. The links to "film sources" and "literature" are especially useful for contextualizing Weimar material.
Expressionist Film in the Weimar Cinema
Expressionism and Caligarisme
Two essays written by "fans" of Weimar cinema and the Expressionist movement. Although they are interesting to peruse, do not rely on them as sources for your research.
Internet Movie Database
The world's largest film information center, this site can be used to confirm things like production histories, casts of characters, initial release dates, etc. Because "fans" are allowed to submit commentary, be wary of what you include in your scholarship.
German Cinema on Video: Resources for Collections
A fine collection of material related to German film from the University of California at Berkeley. The site contains specific links to several figures covered in this course: Fritz Lang, F.W. Murnau, G.W. Pabst, Josef von Sternberg, and Robert Wiene.