In one of our custom Alfresco implementations we are making releases quite often due to somewhat heavy development. The customer used the site notice dashlet to inform users of upcoming downtimes but since there is a lot of sites with a lot of different users this was a quity cumbersome job to do before each release.
To aid the administrators we developed the alfresco-system-messages addon. Its implemented as a datalist using regular share forms when setting up an upcoming message. However, since this is something used system wide we decided to implement it as a Share Admin Console component, placing a datalistcontainer in the regular data dictionary of the Alfresco installation. Messages is time based and have different colours depending on priority.
This addon can be found here https://github.com/Redpill-Linpro/alfresco-systemmessages