This strategy is particularly useful for math and science classes.  I've seen workshops on it, seen it in action in a demo class and seen it in the research (reference below).  I've just not been fortunate enough to have a chance to really implement it to its fullest in my own class.

When checking daily homework, have students mark a little red, yellow or green circle beside each question (or the questions you deem most valuable).  Green implies, "Great, give me a million of these and I'll get 100% on every one of them."  Yellow suggests, "I'm pretty sure I got it.  A little help or a pointer and I'll have it for sure." And red says, "Whoa, am I in the right class?"

Obviously a little prep ahead of time will be necessary to collect up a good number of pens/pencil crayons/felts or you might get sticker "dots" from your favourite business supply store.  I find they're cheap and easy and available in large numbers.

This strategy gets students thinking about their thinking and allows a number of efficiencies since you can take a quick walk through the class and visually assess student competence rather than just address the concepts for those who are willing to stand-out and say, "I don't understand."  It also offers a chance for peer learning.  Students who have mostly green can pair with students with mostly yellow in an A-B Student Talk with reporting out.  Those with a large number of red can have a focussed session with the teacher.  

Paul Black, Christine Harrison, Clarke Lee, Bethan Marshall and Dyllan Wiliam, "Working Inside the Black Box:  Assessment for Learning in the Classroom,"  PHI DELTA KAPPAN September 2004, pp 9-22