Google Voice - Now you can make calls via Google

Google Voice is an application that allows you to enhance the existing capabilities of your phone for free regardless of which type of phone you have or which carrier you are using.

It also gives you,

One Number – The use of a single number that rings you anywhere

Online voicemail – Getting transcribed messages delivered to your inbox
Cheap calls – Cheap calls worldwide and free calls and free text messages to the US and Canada. Get all the info you need here.