I just tripped across an interesting database and redirector service called sipbroker. Basically sipbroker keeps track of sip sites that allow one to call their sip PBX users directly over the net. (Obviously for free.) They also assign a short 3-digit numeric code to each of sip server so one can use them as a redirector service. (This might not be that big a deal for folks running their own asterisk, but it allows folks with on a Sipura (or similar voip adaptor) to also join in the fun too.
main page:list of providers / sip servers: Now the question comes up, which voip service providers that are on the accessible list that people have good experiences with? It seems like the sipbroker idea of cataloging freely accessible sip servers is a good idea, but most of the big players don't want to cooperate. I notice that broadvoice used to play nice, but they decided to withdraw.