Google Wave Add-on for Firefox
Not many people have a Google Wave account, but if you are one of the lucky few, there's a Firefox add-on that's quite neat.Unsurprisingly, it's called the Google Wave Add-on for Firefox, and it basically is a wave notifier. It sits in the bottom right corner of your browser window, and informs you when a new wave has been posted.
Google Wave Add-on
If you drag your mouse over the icon, it pops up a small tooltip with the name of the last updated wave in your inbox.
It's simple but useful.
A word of warning though, this add-on is beta software. While I haven't noticed any problems with it, be prepared for any issues or bugs.
To enter your Google Wave username and password, go to Tools > Add-ons, and there click on the Preferences button for the add-on

Google Wave Add-on preferences
Download here.