No-frills GameObject pooling in Unity. KISS

GameObjectPool.cs

using UnityEngine;

// CustomBehaviour is just a trivial subclass of MonoBehaviour with a bunch
// of static finger-friendly helper-functions :P

public class PooledObject : CustomBehaviour {
	
	// subclass this to treat a behaviour as the basis 
	// of a poolable object.  Stores simple metadata
	// related to pooling
	
	internal GameObjectPool pool;
	internal PooledObject next;
	
	public void PutBack() {
		
		// Return to the pool, if it's still around, otherwise
		// just plain-old destroy this object.  Either way it'll
		// be out of the scene.
		if (pool) {
			pool.PutBack( this );
		} else {
			Destroy( gameObject );
		}
		
	}
}

public class GameObjectPool : CustomBehaviour {
	
	// designer params (forces designers to subclass PooledObject)
	public PooledObject prefab;
	
	// head reference for a plain-old linked-list
	PooledObject firstIdle;

	public PooledObject TakeOut() {
		if (firstIdle != null) {
		
			// activate an old instance
			var result = firstIdle;
			firstIdle = firstIdle.next;		
			result.next = null;
			result.gameObject.SetActive( true );
			return result;
			
		} else {
			
			// instantiate a new instance
			var result = Dup( prefab );
			result.pool = this;
			return result;
			
		}
	}
	
	public void PutBack(PooledObject obj) {
		Assert(obj.pool == this);

		// prepend to the head of the linked list
		obj.gameObject.SetActive( false );
		obj.next = firstIdle;
		firstIdle = obj;
	}
	
	public void Drain() {
	
		// destroy all the idle instances
		while(firstIdle != null) {
			var prev = firstIdle;
			firstIdle = firstIdle.next;
			Destroy( prev.gameObject );
		}
		
	}
	
	void OnDestroy() {
	
		// take out the idle instances when you diiieee
		Drain();
		
	}
	
}

The only gotcha to watch out for is that OnEnable() will be invoked before Start(), but as long as you know that you should be gravy.