A brief summary of some Workflow enhancements plugins.
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
Interesting, but not that exiting Workflow plugin - but its free 
It has one unique facility - the ability to fire a Post-Function after evaluating a boolean - this can be used for "condition" avaluation, here shown in pseudo code:
If {Text} == "A" Fire: Update Resoulution Field to "A"
If {Text} != "A" Fire: Update Resoulution Field to "B"
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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An interesting but not free plugin, one of the very good points being that several post-functions has "Conditional execution of post-functions" - which as the above description enables some conditional execution of the fuction.
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
JIRA Workflow Toolbox
Marketplace: https://marketplace.atlassian.com/plugins/com.fca.jira.plugins.workflowToolbox.workflow-toolbox
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) - 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.
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.