In Confluence Server version 5.8 or upper, now you can install 'Announcement Banner for Confluence' from Atlassian Marketplace.

I've developed a free app which mimics the JIRA's announcement banner behavior.
Feedback is highly appreciated.

This says Resolved, but what version is this change available in? We have downloaded the Public Announcement Banner plugin but it's not supported in 5.10.8

Administrators can create custom HTML an thus make simple announcement banners easily. We do this for example on one of our staging systems (see screenshot)

<div class="warningMessage">This is a QA copy of CAC.  Its content is regularly overwritten with the content of CAC.</div>

Resetting the fix-for-version of issues that have been lingering in the 2.5.x version but have not been fixed yet, and which will have to be rescheduled. Deliberately not pushing them the next version (again), but removing the fix-for version completely for now

Technically, the majority of the code (MessageManager) has been in Confluence since 2.3. Our hosted plugin makes use of it to display closable banners at the top for various purposes. With a little effort, a plugin could be created that uses the core Confluence banner capabilities to allow an Administrator to create and manage such banners.

We have a plugin which has been produced by the Confluence Hosted team which could be used for this purpose.

Oh yeah, just comment out the <div> to hide the banner!

For now, I just use "administration -> layouts -> edit main layout", then add...

<!-- Message Banner -->
<div style="background-color: yellow; border: 2px solid #990000; margin: 4px; padding: 2px; font-weight: bold; text-align: center;">
Your important message...
</div>

...just before the $body tag near the bottom.