The iPhone buzz on the Internet today is the newly available Google Voice for the iPhone. Google Voice, if you're not familiar with it, is an astonishing range of free services. The primary service is the consolidation of all your phone numbers to a single number that Google gives you. That way you don't have to be checking several different voice mail boxes (home, work, mobile phone, etc.). You have a single number that people can call, and you can control which phone rings based on who's calling.
Another advantage is that people don't have to call several numbers to find you. The service provides free voice mail, and you can set it up so that different callers hear different outgoing messages. Google Voice even provides a transcription service such that you can receive the text of voice messages in your e-mail. If you move or change cell phone services, you no longer have to give everyone a new number. You simply change the number in your Google Voice settings.
Actually, it's not an iPhone-specific app that became available but a new Google Voice mobile website that works with several different platforms. You'll need to have a Google Voice account, and since the service is still in beta, they aren't yet giving out accounts to everyone. You can, however, sign up to be on their waiting list. To access the service, you simply point Safari on your iPhone at m.google.com/voice.