I used Gizmo5 and worked (ages ago)

I use now the default SIP client with a voipbuster account.

The people I videochat with need to download on their pc's the x-lite app (ugly, but free and works as advertised)

http://www.counterpath.net/x-lite.html

I have done a lot of videocalls, so I have proof that works.

Hope I've helped!!