Some resources for online test banks are Koofers, Respondus and Scribd. Each site has varying subject matter and varying ways to gain access to the test banks.
Though Koofers requires a registration, access is free. Once signed up, test banks for various subjects are available. Koofers also has a social study outlet, where the class may pose questions and respond in preparation for the test. Study guides, class notes and lecture notes are also available on Koofers.
Respondus is free. However, the stipulation is that the teachers must adopt the accompanying textbook that the test bank is based off. There are options to create online exams and games through Respondus.
Scribd has free test banks that do not require membership. However, more are available through a membership. The first month of membership is free.