The website Piazza.com offers a Q&A forum with minimal overhead. Students can post questions anonymously and I am often surprised how many students take advantage of this opportunity. Additionally, students can contribute answers to questions and instructors can easily see what questions haven’t been answered.
Example: "You can always post your questions on Piazza!"
See this and related tips as a Tip Sheet: http://csteachingtips.org/tips-for-encouraging-help-seeking