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Android only: I like to mute my phone when I'm buried in work, sleeping, or driving my car, but turning on my phone and fiddling with the sound isn't ideal. RD Mute will mute your phone whenever you turn it upside down.
The basic functionality of the app is pretty self-explanatory: whenever you turn your phone face-down, it will turn all the sound completely off. That way you can just flip your phone over and get back to whatever you were doing. This is something that we've covered doing before with Tasker, but it's nice to have it in such a simple app with no setup (and it's free, for those of you that don't want to pay the $5 for Tasker).
RD Mute also has a car mode, which (while seemingly unrelated, but still awesome) mutes your phone while you're driving, so you don't get distracted. You can pick at what speed your phone mutes itself, and even send an automatic text message to your friends so when they call you, they'll know you're driving and that you'll get back to them soon. You can even have it only send the automatic text message to specific contacts if you prefer. Honestly, I'd love to see this feature apply to the regular "face down mute" feature of the phone, too—so I could automatically text my friends to let them know I'm working, asleep, or otherwise unavailable and that I'll get back to them soon (sadly, it only activates this feature if your phone's moving at 31 miles per hour or more).
RD Mute is a free download for Android devices.