This post consolidates a list of impact evaluation resources that I usually refer to when I am asked about impact evaluations. 
This 
cute video explains the factors that distinguishes impact evaluation from other kinds of evaluation, in two minutes. Of course randomization isn't the only way of credibly attributing causes and effects - and this is a particularly hot evaluation methodology debate.  For an example of why this is sometimes an irrelevant debate - 
see this write up on parachutes and 
Chris Lysy's cartoons on the topic. 
Literature on the Impact Evaluation Debate 
The Impact Evaluation debate flared up after this report, 
titled "When will we ever learn" was released in 2006. In the States there also was a prominent funding mechanism which required programmes to include experimental 
evaluation methods in their design, or not get funding (from about 2003 or so). 
The bone of contention was that Randomized Control Trials (RCTs) and Experimental methods (and to some extent Quasi Experimental Designs) were held up as the "gold standard" in 
evaluation.
 Which, in my opinion, is nonsense. So the debate about what 
counts as evidence started again. The World Bank and big corporate 
donors were perceived to push for Experimental Methods, 
Evaluation
 Associations (with members committed to mixed methods) pushed back 
saying methods can't be determined without knowing what the questions are. And others pushed back saying that RCTs are probably applicable in only about 5% of the cases in which 
evaluation is necessary. 
The methods debate in 
Evaluation is really an old debate. Some 
really prominent evaluators decided to leave the AEA because they 
embarked on a position that they equated with "The flat earth movement" 
in geography.  
Here is a nice overview article, (The 2004 Claremont Debate: Lipsey vs. Scriven. DeterminingCausality in Program Evaluation and Applied Research: Should ExperimentalEvidence Be the Gold Standard?)
 to summarise some of it.
Literature on Impact Evaluation Methods
 If you are interested in literature on Evaluation Methods, look at 
Better Evaluation to get a quick overview.