What is BPMS? and when to go for BPMS? is there any free tools for .net or java? is there any plug-in available for eclipse?
It is short for Business Process Management System or Software and basically is a tool for very high level modelling how the business will do whatever they do and how automation tools are integrated.
These things are typically huge and composed of a big collection of tools and infrastructure stuff. Eclipse based tooling may be a part of that, but it is much larger than just a plugin.
If you want to dip your toe in these waters you might look at JBoss Tools which has support for JBPM and Webservices which is a workflow package and the infrastructure which is usually core to these systems.