Software cost estimation is an essential aspect of software project management and therefore the success or failure of a software project depends on accuracy in estimating effort, time and cost. Software cost estimation is a scientific activity that requires knowledge of a number of relevant attributes that will determine which estimation method to use in a given situation. Over the years various studies were done to evaluate software effort estimation methods however due to introduction of new software development methods, the reviews have not captured new software development methods. Agile software development method is one of the recent popular methods that were not taken into account in previous cost estimation reviews. The main aim of this paper is to review existing software effort estimation methods exhaustively by exploring estimation methods suitable for new software development methods.