Mac OS X only: Boxcar is one of the essential iOS apps, as it can notify you about far more than just a new SMS. Boxcar for Mac, just out in beta, does the same thing for your desktop, pinging in for new Twitter replies or DMs, Reddit posts, Github commits — just about anything, really.
Currently, Boxcar is a most excellent app for that straightforwardly pushes notifications from services like Twitter and Facebook to the iPhone. Boxcar 4.0 is going to make your iPhone feel like it has a million, constantly connected tentacles.
