I love net support notify, it's very easy to use, you can send quick messages to all users or target certain users or even teams if you set it up that way. User can't ignore the message and it's massive! The ability to target groups of users makes this product amazing and also the fact you can see which users have seen your message, stops the "I didn't see your message" scenario because you can see they've clicked OK to dismiss the message! This can be integrated with AD and GPO and also be deployed with MSI on GPO which, once you've got the policies correct, means the program is installed automatically on the next GPUpdate. Review collected by and hosted on G2.com.
The only downside was deploying this automatically across the network was a little difficult, however support was very good when I called them and guided me through the best practice for setting the GPO specifically for their product. Review collected by and hosted on G2.com.