• OMG org
• BPMN (Business Process Model and Notation) diagram, how to perform a process
• DMN (Decision Model and Notation) diagram, shows the flow involved in making a certain decision
• CMMN (Case Management Model and Notation) diagram, what is and is not allowed in a process.

These three are part of the BPM+ standard. Charting library: bpmn.io

• UML diagram, similar to BPMN, but uses objects. More focused for IT.
• ARCHIMATE diagram
  • online demo
  • GUI tool
  • Quick guide