I don't trust anything to do with KaZaa.
Anyhow, I use
Free World Dialup. What's cool about it is that it uses true VOIP standards (so you can use a hardware VOIP device if you want). I use the software-based SJPhone to communicate.
BTW, FWD lets you call toll-free numbers (I use the phone cards you posted about with it). So you can call crazy numbers like KDDI's operator line and stuff if you want. And of course it's all free.
Another thing I like is if you wanted to make a really-hard-to-trace call, you could use a proxy, launch FWD, and use a phone card that does not pass ANI to make a call. Even if you use one that passes ANI your caller would get the number from which FWD calls originate and not anything to do with your phone number. Heck I don't even think you'd need to use a proxy.
There's also a service that will let you call to a FWD member from a normal phone line (notaduck.com) but it doesn't seem to work for me. I know that there are other numbers for this that are not from notaduck.com but I've never tried them.
The only thing I find sort of irritating is that a lot of the links on account.freeworlddialup.com don't work, and that includes the link to make a teleconference. I should email them about that...
Anyhow my number there is 628428. I'm not normally on but if anyone wants me to get on and try it, I will.
Almost forgot! A really cool feature of FWD is the "Peering numbers", which allow a FWD user to call VOIP users on other services and vice versa. Unfortunately Vonage's peering got shut down, and it won't call Skype, but it's still neat:
http://www.freeworlddialup.com/content/view/full/333/