you can get a free and hourly updated XML feed from
  http://mise.x25.se/pub/keeptrack/sites.xml

(works with, for example, an XSLT stylesheet like
  http://mise.x25.se/pub/keeptrack/stylesheet.xsl
that would render like
  http://mise.x25.se/pub/keeptrack/ - but hey, use
your imagination instead and write a Firefox
extension or whatever :P)

A true webservice SOAP interface available if
there's a great demand or someone pay me an insane
amount of money (not bloody likely XD)

(yes yes so now I've posted it to the public as
well, are you happy m-chan and will you please
stop bugging me? XD)