We have a basic toaster (systray popup) for Windows that you can install, but it requires some improvements. Specifically, it can be wired into the calendar alarm system. Today it just gives new message notifications (although our web UI automatically polls for new messages, as well). We'd like it to install icons on the desktop and in the start menu. Additional features would be to have a Zimbra search bar.
To download the toaster, log into the administrative interface and click "downloads" on the top right corner of the page. One of your downloads built into the server installation is the toaster app.
For a linux version of the toaster, try ZimbraNotify.