Opus is a new service that aims to help middle school mathematics teachers discover sample math problems aligned to Common Core standards. To find problems on Opus search by entering a topic and selecting a grade. You can also find problems by clicking the "browse the Core directly" link on the Opus homepage. Either way when you find a problem you can save it to your free Opus account where you can then generate a Word doc or Google Document of all of your saved problems. You can also create an answer sheet in your Opus account.
Applications for Education
Opus is still quite new so the collection of problems offered through it is limited, but the idea behind the service is good. If you're a middle school math teacher looking for some practice problems to use with your students, Opus could be useful to you.