I went through a few free apps trying to get a good visual metronome for my Kindle/Android tablet. This is one of the best I've seen and it's non-invasive. No in app ads or extra purchases. Not only gives the click beat, but it does sub beats and flashes the number of the beat you "should" be on. You can also set it for time signatures. On a tablet, it's visible enough at a few feet away to use when piping. This would be a great help to students, too. 5 out of 5 stars.
http://www.keuwl.com/Metronome/