y(n) = (1-p)*x(n) + p*y(n-1)
Unity-dc-gain one-pole lowpass where p is the pole.
A pole close to 1 (e.g. 0.999) will implement a lowpass filter and will "smooth" any control signal.
Simple, versatile but hard to control...
Special case: sound "resynthesis"
And if you're curious, here's more
A "homemade" example: Chris Carlson's Borderland
No JUCE example, but it should be easy to make one starting from this tutorial