The Office of Educational Development at the University of California at Berkeley offers a standard mid-semester evaluation form along with information on how to respond to mid-semester evaluative feedback.
The Center for Teaching Excellence at Cornell University offers a questionnaire that can be administered 3-6 weeks into the semester to assess what components of your course are helping your students learn most effectively.
The Center for Teaching Excellence at the University of Maryland provides two sample evaluation forms that can be used mid-semester. Both are short and straightforward forms containing open-ended questions that will allow you to garner useful information from your students.
The McGraw Center for Teaching & Learning at Princeton University has a student questionnaire form, a student rating form, and a list of possible evaluation questions should you decide to create your own mid-semester evaluation form.