For all those who have GMail accounts and don't use FireFox (i.e. you can't use this hint), there's a free little utility available called GMailStatus. This works just like the "system tray" notifier for PCs that will display the number of messages in your GMail inbox. It also shows account usage, unread messages ... there are quite a few features, as you can see on the above web page.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040928103114679