Jajah, a VoIP provider, has just launched its mobile version, called, unsurprisingly, Jajah Mobile Web.
Like the original web Jajah, there's no download or installation of applications. You simply go to the Jajah mobile site in order to make the call.
You type in the number from which you want to call, whether it be your home number, mobile, or a hotel room, and then type in the number you want to reach.
Your chosen phone will ring, and you simply pick it up to be connected to the person you wish to call.
The beauty of this system is that it even works abroad, so you can make free or low-cost calls from a hotel room far away, although not all countries are covered - large swathes of Africa and South America don't fall under Jajah's plan.
However, most of North America, Australia, and Europe is covered.
Calls to and from Jajah users are free of charge as well, and although you can call to and from mobile phones, this incurs a Jajah charge as well as normal mobile roaming costs.