Qualitative Comparative Analysis (QCA) is a case-based method that enables evaluators to systematically compare cases to document impact. It was first identified by Charles Ragin in 1987, but more recently entered the sphere of policy analysis and evaluation. This excellent resource developed by engineering consultancy Coffey explains, in simple language, how QCA has been applied to a DFID-funded project without a counterfactual.
EvalC3 is another interesting tool for comparing cases. As an excel app, it helps identify sub-sets of attributes that describe an intervention and its context and compare and evaluate the performance of these predictive models.