Gizmo Call is a VoIP service from the Gizmo Project which is backed by the SIP Phone. Unlike softwares like Skype, you need not download an additional piece of software; you can make calls from your favorite browser itself.
The calling service is executed via a Flash widget for which you should download the latest Adobe flash player. You need a headphone though.
Just visit the site and sign up for a username. After signing up, you will be eligible for making or receiving calls. You can call any Gizmo Call, Gizmo5 users or SIP addresses for free. But to make a call to mobiles or landlines, you must purchase a minimum of $10 call-out credits. It doesn’t have affiliate system like Skype to help you earn credits. If you are logged in to GizmoCall, your friends can call you at your username. You can also configure your Gizmo Call account to filter the Correct CallerID number.
A useful Firefox plugin is also available which will make any phone number on web pages clickable and will automatically place a call to that number through Gizmo Call.