The Gmail notifier is without a doubt one of the most useful Firefox extensions to date. It’s so simple, it puts a small icon on the status bar, and it can be set to check your Gmail account every x minutes. When you have new mail, this then pops up a message, which is very similar to the ‘Downloads complete’ message. You can then click on the icon to open up your Gmail inbox.

This adds the one feature to Gmail that it was missing: the ability to check your mail in the background so you don’t have to keep the page open all the time.

Note to Google Labs: Build this feature into the Google Toolbar.

Also, there is a mail client program in development on SourceFourge called GmailerXP (as pointed out to me by Joel). While it’s currently only in Alpha development stage, it has promise. You can follow the news on the project using the RSS Feed, and if you’re interested, they’re looking for developers to help.