Sidehistorik
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The main focusing is primarily on what they contribute on the Post-Function side of a Transition.
Workflow Enhancer for JIRA
Marketplace: https://marketplace.atlassian.com/plugins/com.tng.jira.plugins.workflowenhancer
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Several part of the Post-Functions "Create" or "Add Comment" - I never got them working
JIRA Misc Workflow Extensions
Marketplace: https://marketplace.atlassian.com/plugins/com.innovalog.jmwe.jira-misc-workflow-extensions
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Number 8 and 9 is quite existing and worth examining further.
Also, Number 19 is pretty unique - the possibility to transist linked issues, but this can be scriptet via the Scriptrunner, see here
JIRA Suite Utilities
Marketplace: https://marketplace.atlassian.com/plugins/com.googlecode.jira-suite-utilities
One of my favorites, one of the reasons being that it is free 
| Advarsel |
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Update 2017-10-02: Well, its not free anymore |
JIRA Workflow Toolbox
Marketplace: https://marketplace.atlassian.com/plugins/com.fca.jira.plugins.workflowToolbox.workflow-toolbox
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One of the best on paper (I have not worked with it) - its not free, but its a afordable...
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
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The Scriptrunner is free, and most of the Conditions, Validations and Post-Functions that other plugins on this page offer, can be achived for "free" in groovy. But its all "by hand" - whereas several plugins offer a GUI and other value added fields etc. |
| Advarsel |
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Update 2017-10-02: Well, its not free anymore, since the writing of this article, Adaptavist has purchased Scriprunner. But, its still the best JIRA Plugin by far |
See Scriptrunner tricks here
Essential Custom Fields for JIRA
Marketplace: https://marketplace.atlassian.com/plugins/com.primefieldsoftware.jira.essentialcustomfields
HTTP Request Workflow Function
Marketplace: https://marketplace.atlassian.com/plugins/com.hascode.jira.httprequest-workflow-function
A Post-Function to place a HTTP request agains a URL - an alternative to WebHooks. Reviews seems positive.