Gizmo5 - a combination VoIP and IM client that lets you make free calls

Gizmo5, formerly known as Gizmo Project, is both a service and a software client. The software is available for many platforms, including Mac OS X, of course. Basically it allows voice and video calls and instant messaging between Gizmo5 users. The calls are free as long as you’re calling another Gizmo5 user, and you can purchase credit to use for outgoing calls to regular landline phones. Now, you probably are already using another instant messaging client, so we won’t dwell on that functi