Additional paragraph added at * on 30/1/2013 picking up on an issue raised by Stephen Downes.
Above and below are two screen-shots from the Sutton Trust-EEF's Teaching and Learning Toolkit, which describes itself as "an accessible summary of educational research which provides guidance for teachers and schools on how to use their resources to improve the attainment of disadvantaged pupils" and which "currently covers 30 topics, each summarised in terms of their average impact on attainment, the strength of the evidence supporting them and their cost".
Think if it as an interactive and more practically focused version of Visible Learning by John Hattie; and note the extent to which approaches that are in political vogue in England (like setting by ability, or uniform) are judged to be harmful or ineffective rather than beneficial. For more on the latter, see Ian Gilbert's The Research v The Government.
* As the toolkit explains, average impact is estimated in terms of additional months progress you might expect pupils to make as a result of an approach being used in school, taking average pupil progress over a year is as a benchmark.