This website has colorful balls that bounce according to the noise level in your room. The louder your students, the more bouncy the balls. (Fun part: You can also change the balls to eyeballs, emojis and bubbles!)
Not as exciting as the Bouncy Balls, but a cool monitor that goes from green to red when the noise level gets too loud in your room. This would be good for a small amount of noise to instruct the students to keep their level in the yellow area for group work.
This one isn't a noise monitor as much as a noise controller. You click on the light that you want the kids to work at. For example, no talking would be red. It's a visual reminder on the board for the students to keep their noise level under control.