Not being a Harry Potter fan the "wearabouts clock" has passed me by, but this DIY location-indicating replica is impressive enough even without the back story. The concept is that rather than display the time, the clock's numerous hands show whereabouts each family member is, according to various locations (or situations, such as "mortal peril") around the clock face. As for the magic inside, our old friend the Arduino and a hacked Linksys router monitor Twitter for location updates.
The first generation of the system attempts to recognize the first few words in the tweet - it monitors a different Twitter stream for each hand - and then shuffles the indicators around to the appropriate point. If the message can't be interpreted that way, it points to "read me".
Video demo after the cut