If you mean silence the ringer, check out Foxyring. It allows you to silence the ringer during hours of your choosing. You can also set a list of privileged phone numbers that can always get through. It has the ability to turn off the ringer, or just change ringer volume based on geographic location, though I haven't tried that myself.
My favorite thing is actually a widget that comes with the app that allows you to quickly turn off your ringer for a given time period (up to five hours) and have it turn back on at the end of that time so that your alarm (for instance) works the next morning.