Google Voice support is few and far between on the desktop.  I’ve created a plugin for Digsby which allows SMS messages to be sent and received as if they were sent from the Google Voice web interface.

Messages are sent and received from the phone number itself, contact name support is not enabled yet. To send a new message, use the Digsby->New IM… option, and enter a phone number as 1235554567 or (123)555-4567.

Download DigsbyGV and run the application.  Enter your Google Voice information, and create a password which you will use to connect from Digsby.  Create a new Jabber account in Digsby, with the username voice@localhost, and the password that is set in DigsbyGV.  Once the account is connected, new SMS messages will be routed to Digsby as an IM!

Update: DigsbyGV has been replaced by GVNotifier.net