Software-defined networking (SDN) has become a popular technology, being adopted in operational networks and being a hot research topic. Many network testbeds today are used to test new research solutions and would benefit from offering SDN experimentation capabilities to their users. Yet, exposing SDN to experimenters is challenging because experiments must be isolated from each other and limited switch resources must be shared fairly. We outline three different approaches for exposing SDN to experimenters while achieving isolation and fair sharing goals. These solutions use software implementation, shared hardware switches and smart network interface cards to implement SDN in testbeds. These approaches are under development on two operational SDN testbeds: the DeterLab at USC/ISI/Berkeley and the NCL testbed at the National University of Singapore.