There are a number of very useful add-ons that will provide additional components such as conditions, validators, and post functions. The following presents some of the most popular workflow-related plugins.
Extending a workflow with workflow add-ons
Suite utilities for Jira
You can find a number of very useful conditions, validators, and post functions with this add-on. For example, the Update Issue Field post function that ships with Jira allows you to update any issue fields such as priority and assignee when a workflow transition completes. The Suite Utilities for Jira add-on complements this by providing a very similar Update Any Issue Field post function, which handles custom fields. There are many other useful components...