I made this myself, I wanted to give it a shot and I needed a simple yet customized solution for Camera Dragging.

CameraDragController.cs

using UnityEngine;
using System.Collections;

public class CameraDragController : MonoBehaviour {

	public Transform Target;
	public float Sensitivity = 0.1f;
	public bool Invert = false;
	public bool Hold = false;
	public Rect Bounds;

	private Vector3 TargetStart = Vector3.zero;
	private Vector3 MouseStart = Vector3.zero;
	private Vector3 TargetPosition = Vector3.zero;
	private Vector3 MousePosition = Vector3.zero;

	// Use this for initialization
	void Awake ()
	{
	
	}
	
	// Update is called once per frame
	void Update ()
	{
		if (Hold) {
			HoldDrag ();
		} else {
			Drag ();
		}
	}

	private void HoldDrag()
	{
		if (Input.GetMouseButtonDown(0))
		{
			MouseStart = Input.mousePosition;
			return;
		}
		
		if (!Input.GetMouseButton(0)) return;

		Vector3 pos =  Camera.main.ScreenToViewportPoint (Input.mousePosition - MouseStart);
		Vector3 move = new Vector3(pos.x * Sensitivity, pos.y * Sensitivity, 0);
		
		if (Invert) {
			Target.Translate (-move, Space.World);
		} else {
			Target.Translate (move, Space.World);
		};
		TargetPosition = Target.position;
		TargetPosition.x = Mathf.Clamp (TargetPosition.x, Bounds.xMin, Bounds.xMax);
		TargetPosition.y = Mathf.Clamp (TargetPosition.y, Bounds.yMin, Bounds.yMax);
		Target.position = TargetPosition;
	}

	private void Drag()
	{
		TargetPosition = Target.position;
		if (Input.GetMouseButtonDown(0))
		{
			MouseStart = Input.mousePosition;
			TargetStart = TargetPosition;
			return;
		}
		
		if (!Input.GetMouseButton(0)) return;

		MousePosition = MouseStart - Input.mousePosition;
		
		if (Invert) {
			TargetPosition = TargetStart - (MousePosition * Sensitivity);
		} else {
			TargetPosition = TargetStart + (MousePosition * Sensitivity);
		};
		TargetPosition.x = Mathf.Clamp (TargetPosition.x, Bounds.xMin, Bounds.xMax);
		TargetPosition.y = Mathf.Clamp (TargetPosition.y, Bounds.yMin, Bounds.yMax);
		TargetPosition.z = TargetStart.z;
		Target.position = TargetPosition;
	}
}