Skype is a VoIP program that allows you to make calls over your computer. I started using this about 2 year ago, and I'm quite satisfied with it.
The quality of the calls are OK, but usually the reciever has no trouble hearing me at all.
There are many features, such as a PoGo (Pay as you go) plan, and an unlimited country plan, that really helps if you're a person that's running and business and needs a cheap program to call people.
They also sell headsets on their website as well, which, albeit I have no expirience with, it looks like they're pretty decent.
The plans are very cheap as well, with the unlimited US & Canada being about $3 a month, it may as well replace your current phone. The unlimited world is worth it as well, being only $13 a month, it's a steal.
Skype also has a SMS sending feature, at only $.11 a text, which is really useful, even though normal texts (from a cellphone) are $.12 cents, you can do it from your computer, which allowes you to NOT buy a phone.
Skype credit, their form of minutes, is carried over money to month as well, so whatever skype credit goes unused, will remain in your account until you use it all up. Their pricing for skype credit is also very nicely priced at $10.00 or $25.00.