"It's been like that for a while now ever since You-Know-Who came back into the open. I suppose everybody's in mortal danger now... I don't think it can be just our family... but I don't know anyone else who's got a clock like this, so I can't check."
-- Molly Weasley (HBP5)
Grandfather clock with nine golden hands.
Located in the Weasley living room, this fine clock is completely useless if you need to know the time. Each hand is engraved with the name of a member of the Weasley family . These hands point to inscriptions around the face which indicate where that person is at the moment. Some of the words on this clock are “home,” “school,” “work,” “traveling,” “lost,” “hospital,” “prison,” and “mortal peril” (GF10).
The night that Mr. Weasley was attacked by Nagini the snake at the Department of Mysteries, Dumbledore told McGonagall that Molly might already know due to "that excellent clock of hers" (OP22). Harry realized that the hand representing Arthur would be turned to "Mortal Peril" on the clock.
After Voldemort returned and started his rise to power, all the hands of Molly's clock all turned to "Mortal Peril" (HBP5). That was because Voldemort saw the Weasleys as "blood traitors" - pureblood wizards who were friendly to Muggles and Muggleborns. What upset Molly the most was that they were all in mortal peril even inside their home, the Burrow. She worriedly began to move the clock around the house with her, even while folding laundry (HBP5, HBP6).
Molly also owned another clock which Harry saw on his first visit to the Burrow. It hung in the kitchen and had "only one hand and no numbers at all." It's purpose was to give Molly reminders about her chores: "Written around the edge were things like Time to make tea, Time to feed the chickens, and You're late."
Commentary
Notes
The clock appeared in the Chamber of Secrets film with what appeared to be hands made from silver spoons, each with a picture of a Weasley on it instead of their names.
In the CS book, a different clock was mentioned with no hands, and the official "Molly Weasley Clock" giving the family member locations did not appear until Goblet of Fire.