The Gizmo projects doesn't have the big name that Skype does, but it quite possibly has raised the bar for VoIP calls.
Its newest plan lets you make free calls to both landlines and mobile phone through your computer.
The catch is that the person you're calling has to be an "active" member of Gizmo Project as well.
But there's no fee to sign up, nor any monthly fee; PC to PC calls using Gizmo Project are also free of charge.
Landlines in over 60 countries are covered, although mobiles in only 18 countries are included at the moment.
You also need some sort of headset or microphone/speaker set up to make the calls.
For more information, visitGizmo Project