The Pebble Smartwatch's utility was somewhat limited when it launched, thanks to an API that only supported notifications and data going from your phone to the watch, not the other way around. The last update to the API fixed that, and now Pebble has at least some degree of control over a connected phone. Pebble Phone Ringer Switcher from noted developer Faruq Rasid is the first Pebble app we've seen take advantage of this, at least on the Play Store. It's a simple little watch app that allows the user to switch between normal, vibrate, and silent ringer modes.
To get it working, make sure that your Pebble is awake and connected via Bluetooth, and you've got the official Pebble app installed and set up.