Steve North

Display custom admin notices in your WordPress admin dashboard with native WordPress styling. 
Choose between success, warning, error or neutral styles.
This is a super lightweight plugin that is only loaded on your admin dashboard.

Custom Admin Notice

Does what it says on the tin

Custom admin notices for your WordPress admin dashboard. Display a custom notice to all users in your WordPress dashboard.

Pippin Williamson

This add-on plugin for bbPress will allow you to place forum-wide notices for alerting your users to important information, such as delayed response times, operating hours, new features, etc.
If you have suggestions or bugfixes for the plugin, please report them on <a href="https://github.com/pippinsplugins/bbPress-Notices" rel="nofollow ugc">Github</a>.

bbPress – Notices

An extension for bbPress to easily show notices at the top of all forum pages.

Custom Admin Notice vs bbPress – Notices