Chrome Extension
Download: here
This extension will notify you in your browser when you've been cited, a topic on your watchlist gets a reply, or you get a PM.
Android App
Download: here
This application will notify you when you've been cited or a topic in your watchlist gets a reply. To install it you need to enable untrusted sources in your devices' settings like so:
Unofficial browser extensions
Safari: here. (By stal)
Firefox: here. (By anon)
Use these at your own risk!