One of the benefits HTC builds into many of their devices is a feature that turns the screen off based on input from the phone's proximity sensor. Doing so helps save precious battery life and prevents accidental screen taps (no more "butt dialing"!). Additionally, HTC's implementation allows you to turn your smartphone face-down to not only turn off the screen, but silence your alerts as well. Great for meetings!
Non-HTC phones probably don't have this feature. That frustrated one particular developer, and he decided to do something about it. His app is called Smart Screen Off, and it simply turns your screen off when your phone is in close proximity to something. That something could be a pocket, a purse, or even being set face-down on a table.
You're thinking, hey, why not just hit the lock button? Sure, you could do that, but not only is that two actions, it also puts the phone to sleep. This solution simply turns off the screen (both the display and the touch-sensors). Resuming from that condition is MUCH faster than resuming from sleep. Unfortunately, it doesn't mute the alerts like HTC's solution does.
Even still, it's intriguing, and I gave it a try.
The app comes in two flavors: "Lite" and "Pro". They're basically the same app, but the "Lite" version doesn't let you save any settings. The "Pro" version costs under US$1.50 and provides the following options that the "Lite" version does not:
Start on Boot
Disable when in LandScape mode
Vibrate when screen is turned OFF
Vibrate when screen is turned ON
Hide Notification Messages
Enable/Disable on shake
Don't screen on after screen Off
If you're curious, give the Lite version a try. If you like it, uninstall it and purchase the "Pro" version.