Sidehistorik
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JIRA Suite Utilities
Marketplace: https://marketplace.atlassian.com/plugins/com.googlecode.jira-suite-utilities
One of my (free) favorites, one of the reasons being that it is free
JIRA Workflow Toolbox
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.
One of the best on paper (I have not worked with it) - but not free
Scriptrunner
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.
Several of the plugins above have better or equal as good post-functions (and sometime easier to use) than the scriptunner.