One of the annoying things of Apple's Mail application is that it is hard to detect if new mail has arrived if your dock is not showing. If you have Yahoo's free Widgets installed, you can download the widget Mail Check Star.
This widget will display an unobtrusive red star similar to the one that pops up on top of Mail's icon in the dock. But the best part is you can place this widget anywhere on your desktop, making you aware of incoming mail from any POP accounts in Mail.
[robg adds: I haven't tested this one.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070907160038313