I was really lucky in that I got a Gizmo5 account shortly before they were acquired by google. They created an Android app called Guava that did just that (limited to 3 minutes at a time unless you pay). It comes in handy when I'm traveling internationally as I can make free 3 minute phone calls to the US.

Since Google owns Gizmo5 now and I'm assuming that this new browser calling feature is a result of the acquisition I wouldn't be surprised if an Android app is going to be out soon.