One Phone Number to Rule Them All

GrandCentral is a free service that (tries to) solves the problem of having too many phone numbers. When you sign up, GrandCentral assigns you yet another number, and then allows you to have that number ring through to any number of other phones, or have them all ring simultaneously. It also has web-based voicemail and a number of other call management features, such as the ability to transfer a call from one phone line to another.