There are tons of offers and services available for multiple platforms like Windows, Mac, iOS, Android, Blackberry, Windows phone, etc. You can browse Google to find such services. However, if you don't want to use a service from Google, then the most accessible way is through Skype. Skype is an universal VoIP client that allows free calls from users on the network and paid calls to outside the network at flexible prices. You can check the Rates page for more info.