One that I use is the stoplight. I use this for volume especially during center time.
The arrows control numbers that you can use at your discretion. I use mine to record how many times I have stopped to warn them about their noise verbally. To could also use it to remind the students what their voice level is supposed to be.
I tried this application once. It is meant to work so the students will be quiet and not bounce the balls. Well my students wanted to bounce the balls so they got really loud. It is too bad it cannot do the opposite. I would love to see it bounce when they are quiet as a reward and stop when they are loud.
There are several other resources available so explore!!