I know some people here run their own asterisk servers and others have unlocked ATA's that they can modify the dial plans of. Sipbroker has been collecting the hostnames of open sip servers and assigning a unique prefix code to each of them. That allows anyone, even someone that can't dial alphabetic URL's directly, to dial a three-digit prefix and be redirected to the desired sip server. The effect is that users can use sipbroker to call other sip users directly for free without involving any telco hops.
Folks that run their own asterisk don't really need sipbroker to redirect anything for them. They can just as well have their asterisk server call the destination directly -- if their asterisk knows about mappings. To that end I put together an asterisk config file snippet that can be included in the extensions.conf file to allow asterisk to route calls with a **XXX prefix to the appropriate sip server. These mappings use the same prefixes as sipbroker and route to the same sip servers.
Sipbroker also encourages folks to add listings for their sip servers, so in theory everyone with a sip server could join in the fun.
I'll update these files periodically, so they should track sipbroker's web page as folks add themselves.
-wolfgang