GrowlVoice is a Google Voice client for the Mac

While Google Voice already has a great website, it's severely lacking in its ability to notify you of new messages and let you quickly reply to them. GrowlVoice fixes that.

GrowlVoice quickly notifies you of new messages and provides an easy and intuitive interface for replying to and reading them. You can initiate calls, send texts, and read voicemails all within a couple of clicks. You'll never have to fumble with texting on your phone's tiny keyboard or constantly refresh the Google Voice website while you're at your computer again.