I use F5 a lot on IE and Firefox on a PC. To me it's a standard for "get new stuff." So here's how to easily add it to any application that you want to "get new stuff" within. I'll explain how to do it for Mail.app, but you can then easily add this for any applicaiton (like Safari, etc).
Quit Mail and open System Preferences. Click the Keyboard & Mouse preferences panel, then click the Keyboard Shortcuts tab. Press the plus sign to add a new shortcut key. When the sheet comes down, find Mail in the Application pull-down menu, and enter Get New Mail as the Menu Title. Then click in the Keyboard Shortcut input and hit the F5 key. Finally, click Add to add the shortcut. If you have multiple accounts, you will also want to add one called Get All New Mail, as it is different command (perhaps assign it to Shift-F5). Follow the same steps as above to add this second shortcut.
And then there you have it ... Mail now has one-key mail checking. You can now do the same steps for Safari, or any other application that has a need for "refreshed" information.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060719103254352