Google Voice gives users one number that can ring on all their phones -- mobile, home and office for example -- depending on the rules they set. It includes a bunch of useful and free features, such as transcribed voicemail sent as email or text message, conference calling, outbound domestic calls, and call-screening. International calls are also supported, for a fee, but they cost a fraction of what phone companies typically charge per minute. Alternatively, if you don't want a new phone number to deal with, Google Voice can be used with your existing line -- albeit with limited functionality.
You can check out a brief explanation of the service in the Google-produced video above. Native apps for Android and Blackberry phones are available, while Palm Pre and iPhone users can use the HTML5 web-based app to use most of its features. A full-featured app for the iPhone was also developed, but Apple controversially rejected it last year on grounds that it duplicates functionality in the popular smartphone.