we have recently published a page with FREE (CC non-commercial) mathematical questions. Those questions include random parameters and open answers with automatic evaluation since they are based on WIRIS quizzes.
You can find them at WIRIS collection page http://collection.wiris.com/quizzes/
Notice that if you want to use them you have to install WIRIS quizzes in your Moodle environment http://www.wiris.com/en/quizzes/docs/install
We are looking forward to receive your contributions at email@example.com
All the best, Ramon