I just read today about Gizmo Call, a potentially revolutionary new VoIP offering. Rather than having to install an application (like with Skype), or change over your phone service (like with Vonage), you simply go to their site and use the Flash application to call anywhere in the world. Apparently you get 5 minutes per day for free, and 10 minutes if you register!

Michael Robertson wrote a very insightful post on free calling strategies with Gizmo Call, it's worth a read.