I’ve been using Gmail Notifier for a couple weeks now, and it is growing on me. The default value for checking for new mail is something like 4 minutes, much too long when I’m in the middle of a busy day and have people counting on me to be on top of my e-mail.
You can change the interval using this little trick: hold down the Command (aka Apple or propeller) key and the Option key, then select Preferences… from the Gmail Notifier menu in the menu bar.
A new window will pop up. Enter the values shown:
Click Set, then close the window.