virtualtiers.net (I’m sure that’s not a play on tears!) is a site setup by Jason Grierson (a VMware Communities member) to host practice questions to help study for the DCD exams (510 and 550). Currently there are three design questions, three drag-and-drop questions and fifteen multiple choice questions. Note that the 550 exam (when it’s available again) doesn’t have multiple choice questions but they still good to practice your knowledge on.
At the end of the sample questions you can download a 21-page Word Doc “Study Guide” that covers many of the basic concepts for the exam.
Check back as the site periodically gets new questions added.