This extension allows administrators to create and edit banner campaigns across multiple wikis. Documentation can be found at Help:CentralNotice.
Current cache settings: Banners: 10min server, 0min client; Geo: 0min. Note: Campaigns are not cached - rather, banners assigned to campaigns are cached.
Testing: The Afar Wikibooks project has been closed for some time. However it is still live and can be targeted from CentralNotice campaigns. Alternately you can test on test.wikipedia.org but note that testwiki runs a completely separate instance of CentralNotice.
Building a banner? Need a template to use? Try THIS