Ask HN: Good examples of user notifications in a client-side application?

I'm reworking an application right now, and one of the things that's really been bugging me is how notifications are being handled. Validation errors and exceptions are being reported to the user by pop-up windows, and I think there's a better way to do it. Do you have any examples of applications that handle notifying the user of validation errors / general errors very well? This is a client-server application, not a web application; so that does complicate things slightly. Also, this is a Windows app, so notifications via something like Growl aren't really viable.