WeblogsCom

Dave Winer/UserLand site where WebLog sites ping when they've been updated. (part of BlogWeb/RssAggregator infrastructure)

http://www.weblogs.com/

http://newhome.weblogs.com/faq

implementations (of pinging? of receiving?) http://newhome.weblogs.com/directory/11/implementations

Then you can query the site to get a list of all sits that have been updated in the last 3 hours.