Basic 2D Controller

Controller2D.cs

using UnityEngine;
using System.Collections;

public class Controller2D : MonoBehaviour {

	// Player Movement
	public float moveSpeed;
	public float jumpHeight;

	// No Stick
	private float moveVelocity;

	//Ground Check
	public Transform groundCheck;
	public float groundCheckRadius;
	public LayerMask whatIsGround;
	private bool grounded;

	// Double Jump Variables
	private bool doubleJumped;


	//Player Animation
	private Animator anim; 

	void Start(){
		anim = GetComponent<Animator>();
	}

	void FixedUpdate(){
		grounded = Physics2D.OverlapCircle(groundCheck.position, groundCheckRadius, whatIsGround);
	}

	void Update() {

		//No Stick
		moveVelocity = 0f;

		// Move Right
		if(Input.GetKey(KeyCode.D)){
			//GetComponent<Rigidbody2D>().velocity = new Vector2(moveSpeed, GetComponent<Rigidbody2D>().velocity.y);
			moveVelocity = moveSpeed;
		}

		// Move Left
		if(Input.GetKey(KeyCode.A) ){
			//GetComponent<Rigidbody2D>().velocity = new Vector2(-moveSpeed, GetComponent<Rigidbody2D>().velocity.y); 
			moveVelocity = -moveSpeed;
		}

		// No Stick Player
		GetComponent<Rigidbody2D>().velocity = new Vector2(moveVelocity, GetComponent<Rigidbody2D>().velocity.y);

		//Player walk animation
		anim.SetFloat("Speed",Mathf.Abs(GetComponent<Rigidbody2D>().velocity.x));

		//Player flip
		if(GetComponent<Rigidbody2D>().velocity.x > 0)
			transform.localScale = new Vector3(1f,1f,1f);

		else if (GetComponent<Rigidbody2D>().velocity.x < 0)
			transform.localScale = new Vector3(-1f,1f,1f);


		// Jump
		if(Input.GetKeyDown (KeyCode.Space)&& grounded){
			GetComponent<Rigidbody2D>().velocity = new Vector2(GetComponent<Rigidbody2D>().velocity.x, jumpHeight);
		}

		// Double Jump
		if(grounded)
			doubleJumped = false;

		//Player Jump Animation
		anim.SetBool("Grounded",grounded);

		if(Input.GetKeyDown (KeyCode.Space)&& !doubleJumped && !grounded  ){
			GetComponent<Rigidbody2D>().velocity = new Vector2(GetComponent<Rigidbody2D>().velocity.x, jumpHeight);
			doubleJumped = true;
		}


	}	
	
}

For all the Unity5 users, you can Get rid of typing GetComponent() every time by declaring it as a private Var.so in my script Ive got:
private Rigidbody2D rBody;

then in void start ; rBody = GetComponent();

and then Finally you can reference it like the anim.

rBody.velocity = new Vector2 (moveSpeed, rBody.velocity.y);

Note: Add Physics2D Material and reduce friction to 0.