If you use Google Voice, you must install the official Chrome extension. I hardly ever visit the actual GV web page since I started using the extension. There is one feature I wish Google would implement in the extension, however, and that's desktop notifications.
Until then, I have thrown together a Chrome extension that does just that. I built this from an existing extension, Mini Voice, which shows uses a browser action (icon next to the address bar). My extension shows nothing but a popup notification when a new message arrives.
The extension is really simple and does what it says, but here are a couple of ideas I had for improving it:
Google Voice Desktop Notifier (Chrome Web Store link)
Get the source on GitHub