Many graduate degrees require a test for admission. Some are specific to certain fields (like the LSAT requirement for law school students) and others, like the GRE, can be required for a range of programs. This report by US News provides an overview of the kinds of tests you might need to take before getting admitted to graduate school and includes information about how much exams cost.
Interactive test preparation
Learning Express, online interactive Test Prep materials are available through the New York Public Library and Brooklyn Public Libraries. To access the materials from home you must login with your public library card. (Login required for access)
Find GRE test prep under COLLEGE STUDENTS New York Public Library / Brooklyn Public Library
Sample tests available through Lynda.com at New York Public Library / Brooklyn Pubic Library (search for GRE)