See a current and fairly long list at Wikipedia’s Ping Blog article.
(That article is no longer very exhaustive…)

Here is a new list - please remove any that are no longer current, and add new ones!

http://blogsearch.google.com/ping/RPC2
http://api.moreover.com/RPC2
http://api.my.yahoo.com/RPC2
http://rpc.technorati.com/rpc/ping
http://rpc.twingly.com/
http://rpc.weblogs.com/RPC2
http://www.blogdigger.com/RPC2
http://www.bloglines.com/ping
http://ping.feedburner.com/
http://rpc.icerocket.com:10080/
http://www.octora.com/add_rss.php
http://ping.syndic8.com/xmlrpc.php
http://www.wasalive.com/ping/
http://ping.weblogalot.com/rpc.php

Terry

Category:Advertising
Category: Pinging
Category:EE1