If you go to whirlpoolforums which is a great Australian tech forum, there is a deal going on called "Whirlpool Saver" in the VOIP forum. It will give you an Australian landline number for like $5. They have apps for iOS and Android.

Rebtel has an interesting free deal for its subscribers. Their app will allow two mobiles in different countries to be triggered to call a conferencing number in their respective countries and be bridged together for the cost of outbound local minutes.

Viber mentioned in another thread allows VOIP Over 3g calls between two subscribers.