If you are on a Mac and you use GMail and/or the Google Calendar, Google is now offering a really nice notifier application (it's in beta of course). With the notifier running, you get a little icon in your menu bar (see image below). It's really unobtrusive and extremely useful. It'll give you an indication when you have new GMail messages or upcoming calendar events. It also provides quick shortcuts to create new messages and events. If you're tired of keeping your browser open for GMail and calendar, this is a great little app.

Menu bar with the Google Notifier running