Comment component for Unity3D for storing multiline comments in the editor inspector. Doesn't do auto linebreaks yet (you have to add them manually) but feel free to send me a patch. :)

Comment.cs

using UnityEngine;
using System.Collections;

public class Comment : MonoBehaviour {
	public string comment;
}

CommentDrawer.cs

using UnityEngine;
using UnityEditor;
using System.Collections;

[CustomEditor(typeof(Comment))]
public class CommentDrawer : Editor {
	public override void OnInspectorGUI () {
		SerializedProperty comment = serializedObject.FindProperty("comment");
		string newComment = EditorGUILayout.TextArea(comment.stringValue);
		if (GUI.changed) {
			comment.stringValue = newComment;
			serializedObject.ApplyModifiedProperties();
			EditorUtility.SetDirty(target);
		}
	}
}