I have one installed that lets you toggle between ringer, vibrate, and silent with just one tap for each. It's called SilentMode OnOff by CurveFish (Free) It's not a sliding volume control which is what I think you are searching for. But it is very convenient for silencing your phone right away and also when you tap from silent back to ringer it restores right back to the ringer volume you already had set, rather than you having to slide your volume back up from silent to where you think you once had it set. The button takes up only one icon worth of space on your home screen.
See if it's something you may like.
(They also make a one touch WiFi on/off button as well that I find ultra convenient.)
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