Google Voice is already a useful system, but Google is now expanding their Google Voice offering by now offering the ability to listen to your Google Voice voicemail directly in Gmail.
This new feature works for regular gmail.com users as well as Google Apps Gmail users. To enable this feature in Gmail, you must first have Gmail Labs enabled. In the Labs setting in Gmail, simply enable the Google Voice player in mail setting. All new incoming voicemail in Gmail will allow you to play them directly without having to go into Google Voice.