At work I usually have a Google Calendar tab open in Firefox to remind me of my personal meetings. In Gnome 2 the reminder would flash the task bar and I would notice it. That is no longer the case in Gnome 3, the notification is no longer present. How to fix that?
Google Chrome has this concept of Desktop notifications. AFAIK it is a public spec, but it is not implemented in Firefox and not even being worked on. What a shame. But wait, have a look at this: https://addons.mozilla.org/en-US/firefox/addon/html-notifications/
Unfortunately that doesn’t work due to a bug. But this version works:
http://code.google.com/p/ff-html5notifications/issues/detail?id=32#c6
Now you just need to enable Gentle Reminders in Labs section of your Google Calendar settings. Voila, it works! It leverages native Gnome 3 notifications (which are far from ideal, but much better than nothing). Hopefully I won’t miss my appointments from now on.
It also works for GMail and other web pages using webkit notifications.
Enjoy.
Kparal's Fedora Blog 
I wish this but for Chrome 😉
As I said, it works in Chrome by default.
Try the extension tab notifier
It provides desktop notifications more similar to Chrome’s ones (e.g. more windows are piled up vertically).
Besides, if you want, it monitors titles to update you about sites that don’t support desktop notifications at all.