Android: When installed, the Shush! Ring Restorer app monitors your phone's volume, and when you mute your phone, automatically prompts you to set how long you'd like it to remain silent — so, for example, you don't forget to turn your ring back on and miss important calls.
As soon as your volume hits zero, the screen below pops up on your screen. Drag to adjust the timer, and Shush will tell you what time the ringer will turn back on. Tap and drag on the volume meter in the lower left so adjust the volume Shush uses when it restores sound. Tap the Shush button and you're done.
As the app's Market page notes, it's handy for class, office, movies and sleep. It's simple, intuitive and seamlessly works into how you already use your phone, so it requires almost no extra effort to use. Android apps often get a bad rap (from people like me) for lacking the smart user experience design that you see on a lot of iPhone apps, but Shush is a great counterpoint.
Shush! Ringer Restorer [Android Market]