http://jajah.com/Haven't tried this yet, but if you and the other person are signed up, all you have to do is go to the website, sign in, put in the number you want to call. Your phone will ring and you'll be connected to the other party.
So say you are in New York: sign up yourself and your peeps in
T&T. No more going out to buy a phone card, no more hassle... in theory.
If someone tries it before me let me know how well it works.
edit: Damn T&T isn't included yet. It's 14.2 cents/minute for now.
http://jajah.com/info/rates/ Hope they add it soon.