Too Noisy (free) -- Anyone who has attempted to keep the noise levels under control in a classroom will appreciate this simple, fun and engaging app which displays graphically the background noise level in a room in a fun and engaging way.
My teacher uses a Noise Meter similar to this one. The Noise Meter is a cool way to remind students the correct noise level that they should be using! You just use the paper clip and move it to the appropriate noise level the students should be using.
Chatter tracker--I like this as a reminder to students about the voice level to have during specific times of the day. I wouldn't use this exact phrasing, and I would actually make a stop light to "light up" as a reference for the kids.
My classroom noise-o-meter. I have an arrow that sticks on the board to indicate the allowable level of noise, depending on the activity. Feel free to use or share& me a message if you would like an editable MS Publisher file.