MetaOverflow provides specialised RSS feeds for the Stack Exchange sites. Currently is provides two feeds related to the availability of sites, but more feeds for other things will be added over time.
The application is written in Java, hosted on Google App Engine,
and uses the Stack4J library. It is (currently) closed source. My motivations where:
Writing something useful for myself and the Stack Exchange community.
Gain some experience programming on Google App Engine.
Update 14 Dec 2012
I have finally got around to updating MetaOverflow so it uses that latest Stack Exchange API, so the RSS feeds have started working again.