Mathf.SmoothDamp
static function SmoothDamp(current: float, target: float, currentVelocity: float, smoothTime: float, maxSpeed: float = Mathf.Infinity, deltaTime: float = Time.deltaTime): float;
static float SmoothDamp(float current, float target, float currentVelocity, float smoothTime, float maxSpeed = Mathf.Infinity, float deltaTime = Time.deltaTime);
static def SmoothDamp(current as float, target as float, currentVelocity as float, smoothTime as float, maxSpeed as float = Mathf.Infinity, deltaTime as float = Time.deltaTime) as float
Parameters

current The current position.
target The position we are trying to reach.
currentVelocity The current velocity, this value is modified by the function every time you call it.
smoothTime Approximately the time it will take to reach the target. A smaller value will reach the target faster.
maxSpeed Optionally allows you to clamp the maximum speed.
deltaTime The time since the last call to this function. By default Time.deltaTime.
Description

Gradually changes a value towards a desired goal over time.

The value is smoothed by some spring-damper like function, which will never overshoot. The function can be used to smooth any kind of value, positions, colors, scalars.
	// Smooth towards the height of the target
	var target : Transform;
	var smoothTime = 0.3;
	private var yVelocity = 0.0;
	
	function Update () {
		var newPosition : float = Mathf.SmoothDamp(transform.position.y, target.position.y,
		                             yVelocity, smoothTime);
		transform.position = Vector3(transform.position.x, newPosition, transform.position.z);
	}

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    public Transform target;
    public float smoothTime = 0.3F;
    private float yVelocity = 0.0F;
    void Update() {
        float newPosition = Mathf.SmoothDamp(transform.position.y, target.position.y, ref yVelocity, smoothTime);
        transform.position = new Vector3(transform.position.x, newPosition, transform.position.z);
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	public target as Transform

	public smoothTime as float = 0.3F

	private yVelocity as float = 0.0F

	def Update() as void:
		newPosition as float = Mathf.SmoothDamp(transform.position.y, target.position.y, , smoothTime)
		transform.position = Vector3(transform.position.x, newPosition, transform.position.z)