I don't know New Zealand providers, but this site will help you: http://www.prepaidgsm.net . The forums are quite useful too.
I use a UnitedMobile+ SIM which permits free incoming calls in about 80 countries in the world. The outgoing rates are about 40 cents a minute in New Zealand. (Unfortunately the inbound and outbound rates in North America are obnoxious, but I use Rogers in Canada and T-Mobile in the US.)
SIMs are my other weakness. I have mobile numbers in the UK, Austria, Germany, Switzerland, the US and Canada.