RSS over instant messaging
Web app immedi.at takes a RSS or Atom feed specified by the user (selected via a small bookmarklet) and then sends alerts to the IM application of your choice when the feed is updated. Currently AIM, MSN Messenger, Jabber and ICQ are supported.
At first glance, the idea is awesome, although it might get a little bit overkill with some more popular sites - say Slashdot or BBC News, but it does do exactly what it says on the tin.
However, the killer app might just be a combining it with a keyword based feed such as my earlier Yahoo! News over RSS article, alerting you to stories from multiple sources based on topics of your choice.
Currently the service appears to pick up its feeds via autodetecting the URL's in the selected site's meta tags, but with a little tweaking it might not be too difficult to point it in the direction of a XML file. (The system is very much in beta right now, so the functionality might well appear in the coming weeks nonetheless.)