Life-timer
Every creature or entity with a finite lifespan has one. As the only true immortal is Death (who has a stylish black life-timer completely empty of sand) this means that the shelves in Death's Domain are groaning with life-timers for everything else.
Most mortal beings have the standard-issue egg-timer look-alike, through which the sand only pours one way regardless of whether or not it is upended. People can apparently judge how much life they have left from seeing their timers, since when Death arrives for Renata Flitworth in Reaper Man she tells him she knows why he's there - 'I saw how much sand was in that timer'. (Death himself received a golden life-timer as an ironic parting gift when "retired" by the Auditors. This is analogous to the famed gold watch given to the Roundworld retiree who has spent all his working life in the same job.)
However, a carefully hidden Special Room houses what might be called the VIP lounge. The selection here covers Gods, demons and anthropomorphic personifications. In pride of place is a VERY large life-timer which has a delicately engraved turtle on the glass. This hidden room is not kept as secure as it should be. Susan discovered to her horror that cats had got in there (a situation possibly engineered by Mr Teatime?) and the floor was covered in shards of broken glass which turned out to be from the Hogfather's life-timer. Subsequent events are recounted in Hogfather.
For creatures whose existence is powered by belief, such as the Hogfather, the amount of sand in their life-timer reflects the belief the inhabitants of the Disc have in them. When Death breaks the Hogfather's life-timer, it prevents him from dying as the 'maybe' can no longer flow into the 'was', keeping his level of belief in stasis and allowing Death to fill his role.
Some interesting life-timers include that of Scraps, Igor's dog in Carpe Jugulum. It's misshapen and bumpy, presumably indicating the dog's artificial life-extensions. Igors probably also have similar life-timers, because they are also made up of "spare parts".
Rincewind's life-timer is described as resembling something created by a glassblower with the hiccups in a time machine, due to his being hit by so much magic and being forcefully shuttled through time and space so many times. Based on the amount of sand, Rincewind should have died long ago, but the shape of it is such that the sand often flows backwards or diagonally, with the result being that even Death has no idea when Rincewind will die. This represents something of a curiosity to Death, who keeps the hour-glass on his desk as a paperweight so he can keep an eye on it. Albert's life-timer is unique. Mugged on a rare visit back to the living world and Ankh-Morpork, his official life-timer - which he insisted on keeping himself - got broken. The Death of Rats collected maybe ninteen seconds worth of life from the wreck, and this is now stored in a beer-bottle. Technically, Albert is not immortal. but those nineteen seconds, unless interrupted, will go on for ever. They say time is elastic...
Nanny Ogg also has a unique life-timer. Given as payment for delivering the sons of Wen the Eternally Surprised, it contains a day's worth of time and has Tempus Redux (Time returned) written on it. That might come in handy...