Really, really simple web app with three columns: green, yellow, and red. Oh, and counters at the bottom of the page, so you can see how many times the columns have been clicked. Oh, and a "Reset" button.
That's it.
Currently it will probably look like shit on anything else than an iPad-sized device, since there are a couple of images whose size have been hacked quickly to fit. Use and modify at will.
If you're going to use it for anything, though, take the files in this repository and put them on your own web server, since I won't take any responsibility for the current state of the code.
Hosting a conference and want feedback on the sessions? Put some tablets with this app at every exit in the room.
Owning a restaurant and want feedback on the customer experience? Put up a tablet next to the entrance.
In general, this simple app is perfect for getting people to provide rough feedback. Rough feedback is always better than no feedback.
So this is a web app, which means you need to fire up a web browser and load the page. Most mobile browsers will hide the browser bar if you scroll down a little, which is why the three columns are a little more than 100% high.
However, if somebody accidentally scrolls up instead of clicking one of the colors, the browser bar will reappear. That may be annoying.
Consider using some kind of kiosk browser, like: