Extension:DismissableSiteNotice

The DismissableSiteNotice extension allows users to close the sitenotice, using cookies. When you add a new sitenotice and want everyone to see it, change the number on the created page MediaWiki:Sitenotice id by one (e.g. if it would be 5, you'd replace the page with the number 6 and so on).

Installation
To install DismissableSiteNotice, download the latest snapshot and extract it to your extensions directory. Then, add the following line near the end of your LocalSettings.php:

Customization
The default dismiss button takes up 20% of the page and is centered. To move the dismiss button to the top right, and have it not take up a lot of space change the following:

Modify DismissableSiteNotice.php. Remove lines 65 and 66 and add everything below. document.writeln(' '); document.writeln(' '); // May need to modify img src to where ever your installation is located. document.writeln(''); document.writeln(' '); document.writeln("$encNotice"); document.writeln(' ');

Download the image found here: http://bits.wikimedia.org/skins-1.17/common/images/closewindow.png and add it to the DismissableSiteNotice folder.

meta.wikimedia.org and wikipedia have similar setups.