I recently came across GTalk-to-VoIP, which is a free gateway allowing voice and IM communication between Google Talk, all SIP services, Yahoo Messenger, MSN Messenger and the PSTN. For example, I can use it to call Google Talk users from my SIPphone account, and Google Talk users can use it to call me or (for a reasonable price) phone numbers on the public telephone network. It also has lots of other features, and there is plenty of information on the site. I recommend having a look if you have any interest in voice communication.
