1. Big Switch - Their current solution is built around Openflow but they supply both the switch OS and the controller in order to build out a solution. They do allow you the choice of hardware from certified vendors. Switchlight is built on the Open Networking Linux (ONL) which is part of OCP.
2. Cumulus - This solution is built around the Linux OS which allows you the flexibility to develop applications that is tailored to your network needs.
3. Pluribus - The concept of a network fabric does allow ease of management and therefore allows more predictability for application deployment especially using the fabric API concept.
4. Pica8 - Openflow focused switch OS but does have a very mature traditional protocol suite.
Those are the main players today in the open networking arena as independent switch OS providers. The question that comes to mind is; How well does these different offerings play together? This is where standards could play a better role over time. The customer has choice in hardware but today it would be difficult to pick multiple switch OSes and get a clean infrastructure management story.
One good narrative is to standardize on a protocol like Openflow then settle on say Open Day light (ODL) as the main foundation for a controller. Once that is in place, then vendors would focus on delivering applications as their main differentiators. This will probably not happen any time soon but there maybe a case to build out open standards to real enable choices.