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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.