For a long time I've been spending most of my time (and the little money I have) developing an app for Android smartphones – beeZee.
It's pretty simple and it's free so please try it
The app is useful whenever you are busy (that's where the name beeZee came from..), when you can't answer for some reason (at work, driving, at class etc.)
Just select your current status (the reason you're busy) and when someone calls you and you can't answer, beeZee will automatically send a text message explaining why you're unavailable.
Most importantly, I would like to get feedback from you guys so I can make beeZee better.