I currently use an add-on that allows you to manually create an alert and send it using various criteria, including user groups - this is fantastic for people who are involved in particular activities (like roleplays), but I think it this should be core, and also work from the front end.
So it a nutshell what I am suggesting is:
- Allow manual alert to be sent, entering text and optionally a link
- Allow manual alert to be sent on various criteria including usergroups
- Allow manual alert to be sent via the front end by moderators (if permissions allowed)