Not all OER texts have ancillaries (test banks, exercises, etc.), but many do, and will be available with the text. If your chosen text doesn't have ancillaries, or you're interested in using ancillaries to support your course, in general, check out Merlot, do a Google search, etc.
Many commercial and open publishers or websites are offering online homework systems that line up with open textbooks at no-to-low cost. OpenStax, in particular, will list suggested partners with the appropriate textbook.
The MERLOT system provides access to curated online learning and support materials and content creation tools, led by an international community of educators, learners and researchers.
Are you an instructor who wants to adopt an open textbook, who feels online interactive homework is valuable, but doesn't want their students to have to pay an additional fee? Then read more about using MyOpenMath in the classroom.
This website provides training and tools to help you solve statistics problems quickly, easily, and accurately - without having to ask anyone for help.
GNU PSPP is a program for statistical analysis of sampled data. It is a free as in freedom replacement for the proprietary program SPSS, and appears very similar to it with a few exceptions.
An open statistical package sponsored by the University of Amsterdam.
Ximera lets you write documents in LATEX and post them as interactive activities online for your students.
Desmos Classroom is a free teaching and learning platform for graphing and other quantitative techniques (scientific calculator, graphing calculator, matrix calculator, geometry tool, 3D calculator, and basic math functions).