Android: Not all text messages are equal. Some require immediate attention, but you don't know until you check. TeXTe allows people messaging you to put an emergency-level alert on certain messages with a simple text-based trigger.
The app requires you to set up a trigger phrase that will set off an obnoxiously loud noise whenever a text message contains said phrase. You have your choice of a few different ringtones, including a fire alarm and a rather perturbed goat. The alert will sound even if your phone is in silent/vibrate mode. Of course, this has a lot of potential for abuse, so be careful who you trust with your trigger phrase.