GVJava is a program I started before Google Voice offered forwarding texts to gmail. The program sits in your system tray and notifies you with a bubble native to your platform. Windows will use the standard info bubble in the system tray and OS X will use a growl looking bubble in the top right on the screen. Unfortunetly, I couldn't actually add growl support because Apple doesn't allow Java to make cocoa calls. I know there are ways around it (such as rococoa), but I couldn't figure it out.
When you first run the program, GVJava will greet you and ask you to login. Use your standard Google username and password to sign in and you're ready to get notified. If you are behind a proxy, you can easily enter in the ip and port, and GVJava will work fine. You will be notified of unread texts, missed calls, new voicemails, but does not support viewing or replying to these messages. Google doesn't have a GV API so just getting the notifications to work was hard and can easily break if Google changes anything.