I find the Google Notifier to be invaluable, since I don’t get a large enough volume of mail to run a native client while at home. A number of months ago, the rate at which it checked for mail decreased significantly.
Luckily, there is a simple fix:
defaults write com.google.GmailNotifier AutocheckInterval # (where # is the number of minutes between checks)
Just jotting this down so I don’t lose this. Adapted from the original instructions on MacOS X Hints.