Android Google Voice lets users make calls or send SMS from their mobile phones using their Google Voice phone number. But the more exciting feature is the ability to check your Google Voice voicemail on a mobile device, complete with automatic message transcripts.
Of course, you don't need a mobile app to have all calls made to your Google Voice number directed to your cellphone or any other phone. But when you add that feature to the mobile client, Google Voice is starting to look like a pretty full-featured service for managing your phone calls and text messages on a mobile device.