Wikipedia has five kinds of top-of-page messages that can be used to convey information or announcements to readers and editors.
The "Sitenotice", found at MediaWiki:Sitenotice, is displayed at the top of all pages for all logged-in users, and for anonymous users if the following message is empty.
MediaWiki:Anonnotice can be used to display information only to anonymous/IP users, not logged-in users. Alternatively, by 'blanking' the anonnotice and replacing it with nothing (not even a space or empty paragraph!), then the sitenotice can be used as a "logged-in users only" notice to display information only to editors. When "Anonnotice" contains nothing, no notice is shown to anonymous users. When it contains "-" it disables the message in favour of Sitenotice. If it contains anything else, it will be rendered to anonymous users instead of the Sitenotice (including a "hide" button).
The CentralNotice, found at meta:Special:CentralNotice, is an extension which can display banners at the top of a select group of Wikimedia sites simultaneously.
Geonotice, at Wikipedia:Geonotice, allows location-specific notices – and is a candidate for integration into MediaWiki.
In order to place a notice at the top of every registered user's watchlist, an editor can make a request at Mediawiki talk:Watchlist-messages. Once a consensus forms to list a notice, it is added, and the notice counter (the cookie-ID) is incremented. Notices should be approved for a fixed length of time after which point they will be removed.
Adding the template {{Edit protected}} to a request will ensure that an administrator is aware of it. This is particularly useful for uncontentious changes, such as correcting a typographical error in an existing message.
