Draytel http://www.draytel.org/ offer the ideal SIP service, no fees, no monthly subscriptions, free incoming geographic number, outgoing call rates are reasonable. The snag for most on this forum is that the incoming geographical numbers are UK only.

This http://www.voxalot.com/action/home is interesting, lists VOIP (SIP?)providers in respective countries and rates them by Call Quality, Support etc. If nothing else it is a useful list of sip providers in your location along with an assessment to aid evaluation.
Voxalot claim to allow access to all of them. I need to look more into this service when I have a bit of time.