Network virtualization is seen as an enabling technology for the Future Internet. In this context, many Virtual Network Embedding algorithms have been introduced in literature so far. This paper discusses an open source framework for the evaluation of such algorithms. The paper describes features provided by the framework, how to use the framework for evaluating these algorithms, and how to extend the software with respect to novel algorithms and simulation scenarios. Lessons learned are presented, describing how the software evolved towards a mature and highly extensible simulation framework.