MDE (Model Driven Engineering) is an approach to software engineering centred around modeling. Code generators are an important component of the MDE toolset because they bridge the gap between models and their implementation. Studies have demonstrated that deficiencies in MDE tooling are a common cause of failure in MDE adoption, highlighting the need for research to identify and address inadequacies. This paper has two contributions. First, it performs a survey of Open Source tools dedicated to special purpose code generation. Second, it identifies a core set of properties that characterise their approach, which we argue have application to code generation in MDE. These properties can be leveraged to guide new tool development as well as for evaluating existing tooling.