Here's a short list of some of the better ping services that you can set up within XOOPS modules that ask which services you'd like to ping with updates to your site(s).

http://rpc.blogrolling.com/pinger/
http://www.blogdigger.com/RPC2
http://rpc.technorati.com/rpc/ping
http://rpc.britblog.com
http://pingoat.com/goat/RPC2
http://bulkfeeds.net/rpc
http://rpc.weblogs.com/RPC2
http://api.my.yahoo.com/RPC2
http://services.newsgator.com/ngws/xmlrpcping.aspx
http://api.moreover.com/ping
http://api.moreover.com/RPC2
http://www.blogpeople.net/servlet/weblogUpdates
http://www.blogoon.net/ping/
http://rpc.tailrank.com/feedburner/RPC2
http://ping.feedburner.com
http://ping.syndic8.com/xmlrpc.php
http://ping.weblogalot.com/rpc.php
http://pinger.blogflux.com/rpc
http://blogsearch.google.com/ping/RPC2
http://rpc.icerocket.com:10080
http://rpc.pingomatic.com
http://api.feedster.com/ping.php