Jelly is not reserved just for CMS. Since the ServiceNow platform is built using the language, there are several places where you can inject your own code to further control your instance.
One of the simplest ways to do so is with a Formatter. This item provides a mechanism to include a UI Macro onto a form. Navigate to System UI > Formatters to see the out of the box items. There are several examples, including the Process Flow formatter, the Activity Log, and the variable editor placed on the ITSM tables, that show what is possible.
Let's cover a simple but useful example. This will run on the Maintenance form, making it more obvious when an SLA has breached.
Navigate to System UI > UI ...