Going on with the list of custom actions started on List of Nintex Workflows Custom Actions - Part 1 , here you can find more actions of any kind for Nintex workflow.
You are free to participate on these post collaborating with this list.
Simple and useful. This custom action is capable to run powershell commands.
Collaboration Greatly Simplifies the Routing of Incoming Faxes into Nintex Workflows and the Sending of Outgoing Faxes from Nintex Workflows
Not free. Unknown
A PDF Converter for SharePoint ships with a large number of Workflow Actions for both SharePoint Designer and Nintex Workflow 2007, 2010 and 2013.