Sidehistorik
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Marketplace: https://marketplace.atlassian.com/plugins/com.tng.jira.plugins.workflowenhancer
Interesting, but not that exiting Workflow plugin - but its free 
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An interesting but not free plugin, one of the very good points being that several post-functions has "Conditional execution of post-functions" - wich which as the above description enables some conditional execution of the fuction.
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Marketplace: https://marketplace.atlassian.com/plugins/com.googlecode.jira-suite-utilities
One of my favorites, one of the reasons being that it is free
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Marketplace: https://marketplace.atlassian.com/plugins/com.fca.jira.plugins.workflowToolbox.workflow-toolbox/versions#b400
This has some very good field post-fuctions for handling lists and formatting text fields.
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Marketplace: https://marketplace.atlassian.com/plugins/com.onresolve.jira.groovy.groovyrunner
An excellent and free plugin giving the developer at lot of options and possibilities to enhance the solution. Some very complex groovy stuff can be achived here.
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