GrowlVoice can either notify you through Growl (http://www.growl.info), or through "basic" notifications that require no third party software. To reply to a notification, just click it.
If you have unread messages, GrowlVoice displays the count succinctly within its icon in your menubar.
In GrowlVoice's drop down menu, you can view the unread count for your inbox, texts, voicemails, and missed calls. Clicking on one of them will take you to the respective Google Voice inbox in your browser.
A Google Voice account is required to use this app.
To ensure that GrowlVoice is able to alert you of missed calls, enable "Place missed calls in the inbox" in the "Calls" tab in your Google Voice settings.