The debug log that shows the output of Business Rules is very useful to display which Business Rules are running and when. To help with the client-side, there is a basic JavaScript watcher lets you see output from Client Scripts.
Tip
The JavaScript debugger that was in previous versions of ServiceNow allowed you to edit Business Rules, set breakpoints, and inspect JavaScript variables during execution. This functionality has been removed in the Helsinki version, and will be revamped for the Istanbul release.
The alert box is probably the most often used way to display information. It's probably also the worst! It interrupts and distracts the user, and the message isn't stored or logged. In addition, for a user, it can be difficult to cancel multiple invasive popups.