Place a widget on top of every site's dashboard under the same Multisite installation, containing whatever content the Super Admin writes.
NOTE: ONLY works with multisite (aka WordPress Network) mode for now.
A super-easy-to-use Multisite WordPress news broadcasting tool for the Super Admin of the Network.
This plugin can be helpful for the Super Admins to push a notification, some kind of messages, even some Ads if they like, to every site admins who registered under your Multisite WordPress network. The added widget will go to the first place of all widgets, unless the site admins move it elsewhere.
This is a very simple task but yet no plugins ever met my simple need, so I made one for myself, and hope it helps you too.
You can find the configuration page at Manage Network -> Settings -> Dashbaord Broadcast.