Have been enjoying playing around with the BBC's new Torchwood ARG, another example of the BBC treating the web as a creative canvas (disclaimer: I work for BBC Vision Multiplatform, although wasn't directly involved with this commission).
Whilst I'm not sure if it wholly conforms to the received wisdom on what an ARG is, it plays nicely off the broadcast narrative, makes good use of embedded video (clip below) and weaves in a constellation of satellite sites (e.g. New Eden Tech, Venus Clinic and Dark Talk, which also has a MySpace page).
The only downers are the rather clumsy disclaimers (see screencap from Matt Jones) and the rights limitations which mean that you can't view the main content outside of the UK (international visitors are redirected to this page). Google for 'Torchwood ARG' and you'll see that much of the discussion is around the geo-blocking rather than collaborative problem solving, which feels like a shame.