Invites have only been out for a few short days, and already Google Voice has an add-on. Google Voice is a service that consolidates all of your phone numbers (including Skype) together under one number. It's free (you'll have to wait a long while for invites though) and you can even pick vanity numbers.

That's an especially attractive proposition for small businesses, as it would allow them to have a custom number for their company that links multiple cell, in-office, or sky numbers together. Unfortunately, Google Voice currently lacks any ability to allow you to differentiate between which number which call goes to. This would be a major limiting factor for the service, if the good people at On-State hadn't stepped in.

They have a program called business presence. What it does is add an extra layer to the calling process that lets it choose which number to select based on a number of criteria. It can choose to decided based on which workers are present, which ones are at work or in the field, and a variety of other factors. Employees just have to install a simple program that lets them select their availability status in order for business presence to prioritize them. Business presence doesn't just work for voice. Skype, Google Talk, and a number of other options are available.

The fact that other company's are seeing the usefulness of Google Voice so quickly bodes well for the future of the service. An enormous number of people have already signed up for the beta test, and I have every expectation that it will become at least as popular as Gmail is eventually. It's just way too useful to not succeed.