While not 100 percent perfect, this ranking system and database from MyVoIPProvider.com, is a huge eye opener when it comes to seeing what's out there in the way of competition.
While also not complete, they list 516 suppliers, the listings and ranking based only on cost, but it is a starting point. I'm not in the cost savings sector when it comes to VoIP, as research by Level3 and Yankee Group is starting to show. I'm more a believer in features and applications, use of open standards and empowerment to the end user, not only enabling the service provider to be in sole control of a users telecom options.
In looking this list over it's a boon to owners of PhoneGnome and also to users of soft clients like EyeBeam from CounterPath. Now you can really find a range of service providers in one easy place.
Combine this with SIP Broker's Provider White Pages