Desktop notifications for emails and chat messages were announced on the Official Gmail Blog yesterday. Though not life-changing, this is a feature that I am glad to see.
Gmail is typically open as a tab in my browser at all times, even while I work on non-email related items and tasks. This means that I regularly miss others' attempts to chat with me. By the time I realize they have sent me a message and attempt to respond, they may not even be online. If this bothers me, then I'm sure it's even more frustrating for the person initiating the conversation. Problem solved. The best part is that I don't need to run an application in the background (such as iChat or Adium) in order to receive chat notifications any longer!
I'm less excited about the email notifications. I prefer to control my inbox and not the other way around. Email notifications kill my productivity, so I only check email at designated times. There is an option to be notified only when "Important Messages" arrive, however. As a Priority Inbox lover, I may consider giving this a try.
Desktop notifications are available only to Google Chrome users at this time. Details here.