One of the most popular expressions for After Effects. It allows animate virtually any parameter with a damping effect.

amp = .04;// The higher the value, the greater the amplitude
freq = 2;// The higher the value, the higher the frequency
decay = 5;// The higher the value, the smaller the delay

n = 0;
if (numKeys > 0){
n = nearestKey(time).index;
if (key(n).time > time){
n--;
}
}
if (n == 0){
t = 0;
}else{
t = time - key(n).time;
}
if (n > 0){
v = velocityAtTime(key(n).time - thisComp.frameDuration/10);
value + v*amp*Math.sin(freq*t*2*Math.PI)/Math.exp(decay*t);
}else{
value;
}