I've given pre- and post-tests, collected them (unnamed and unerased), categorized response types and reported to the class their progress per category graded.  For a college-level physics class of pre-professionals, students appreciate the low pressure quizzes every once in a while.  This kind of formative assessment demonstrates to students that you really are interested in their learning, too.

Since I don't "mark" the boards for a grade, evaluating them doesn't take very long.  And, they can actually be interesting and fun to assess.  Did I really just say that?

I'll occasionally keep some boards for class discussion the next day.  I don't ridicule responses, but I do point out typical challenges people new to physics face (free body diagrams, for example).  Students like seeing their work shown as examples of "job well done."

So how do you "whiteboard" in your classroom?