Software Defined Networking is a new and exciting development in networking. However, it's been a long time coming:
1985 IGRP planned to take into count metrics beyond hop count to include reliability and load
1991 OSPF specified calculating paths to deliver better throughput, lower latency, etc. John Moy's excellent book remains one of my very favorites.
1996 Cisco Policy Based Routing was introduced with IOS 11.0 and "provides a flexible mechanism for network administrators to customize the operation of the routing table and the flow of traffic within their networks."
2006 Cisco OER became PfR sometime around 2010. It allows the traffic forwarding decision to be based on delay, packet loss, and link loading. Arguably the closest precursor to SDN (at least from Cisco) in that it has a 'separation of control plane', 'a centralized controller' and possibly 'programmability by external applications.
Please send me other examples!