It seemed to me that it is useful to have a list of the books by subject matter, particularly if you want to introduce someone else to Pratchett. This is based on U169755's entry about the Discworld books.
This list excludes spin-off books, maps, calendars, etc. I am a fan, not an obsessive. Ok?
The First Two Rincewind Books
The reason for listing these separately is that these first two books are noticably different from the later ones, being more random and some would say more fun.
The Colour of Magic (18 January, 1985)
A plotless romp which parodies almost every sword and sorcery book you have read, and many that you haven't. Rincewind the Wizzard meets Twoflower the Tourist and his Luggage as gods play games with the lives of mortal men.
The Light Fantastic (5 September, 1986)
Slightly more plot as Rincewind and Twoflower meet the elderly Cohen the Barbarian as the Discworld's sun heads on a collision course with the Discworld.
Other Ricewind Books
Sourcery (June 1989)
A Wizard is an eighth son of an eighth son. If you take it to a third generation and you get Sourcerers, the source of magic itself. This is a Bad Thing, and the reason why wizards are not allowed to marry. Usually.
Eric (not Faust)1
(Also published as a Graphic Novel). Eric, the Discworld's youngest demonologist summons a demon, but gets Rincewind on a crossed line. Rincewind then has to grant Eric three very adolescent wishes.
Interesting Times (November 1995)
Rincewind finds himself with the Luggage in a place which isn't China. There he meets Cohen and his geriatric barbarians the Silver Hoarde who are going for the biggest theft of all - they are going to steal an entire Empire.
The Last Continent (1 May, 1999)
Rincewind has ended up in land known only as Four-Eks. Meanwhile Ridcully and the Faculty of the Unseen University arrive on a distant shore 10,000 years before. Is it any wonder that rain is a distant memory of the Dreamtime?
The Last Hero (November 2001)
Illustrated by Paul Kidby
The last outing for Cohen the Barbarian and the Silver Hoarde who decide to return fire to the gods. With knobs on.
Equal Rites (13 November, 1987)
Introduces Granny Weatherwax, who grows and develops as the series progresses. She takes the eighth son of an eighth son to Ankh-Morpork to enrol as a wizard. The only problem? The child is girl.
Wyrd Sisters (10 November, 1989)
The first outing for all three witches of Lancre: Granny Weatherwax, Nanny Ogg and Magrat Garlick, who take on royalty and actors in a very scottish play.
Witches Abroad (12 November, 1992)
Every fairy story has a fairy godmother and a wicked witch, right? So if Magrat is the godmother, what does that make Granny Weatherwax? The three witches travel to the land of Genua via every fairy story you can think of.
Lords and Ladies (4 November, 1993)
Granny Weatherwax knows that Elves are not the gentle folk many take them for. They are evil and spiteful. Unfortunately most of the people of Lancre imagine that they are delicate, beautiful and glamorous. It is midsummer, it is night, but it isn't a dream.
Maskerade (7 November, 1996)
A night at the Opera in Ankh-Morpork. Agnes Nitt has the voice of an angel and the body of... well, at least she has a wonderful personality. But who is that mysterious masked phantom? The show ain't over till the fat lady sings.
Carpe Jugulum (4 November, 1999)
Lancre is visited by a family of Vampyres (note spelling) and it would seem there is no defence against them once they have dealt with Granny Weatherwax.
Death, and his Grand-daughter Susan
Death appears in every single one of the Discworld books. These are the ones in which he is a major character.
Mort (November 1988)
Death is worried about his daughter. Sixteen is such a difficult age, and she has been sixteen for centuries now. He hires and apprentice, Mort, and takes some time off to experience life and poker (he has the perfect poker-face, after all). Unfortunately things do not go to plan.
Reaper Man (21 May, 1992)
Death decides to take a break. He takes a job on a quiet farm. In the mean time no-one is dying and people are getting upset.
Soul Music (4 May, 1995)
Music with Rocks in arrives on the Discworld and its got a beat. Soon everyone wants to be a rock star, including the Faculty of the Unseen University. Death's grand-daughter Susan really manages the gothic look.
Hogfather (1 November, 1997)
How do you assasinate an anthropomorphic principle? It is Hogswatch Eve, and the Hogfather is missing. Death's grand-daughter Susan steps in, and there is blood on the snow before morning.
The Thief of Time (1 May 2001)
The Auditors want to stop Time: it's messy, and it is much easier to count things like atoms if they aren't moving. They know how to do it, too. Susan and the History Monks are in a race against time, or to save time, or to get there in the nick of time.
Samuel Vimes and Ankh-Morpork
Guards, Guards! (16 November, 1990)
Introducing Samuel Vimes and city watch of Ankh-Morpork, and based on every film noir movie you have ever seen. The watch have to deal with a Dragon which may or may not exist, but which is definitely burning down the city.
Men at Arms (3 November, 1994)
New members of the Watch include a dwarf and a troll while assassins clown around with an explosive new weapon.
Feet of Clay (1 April, 1997)
Samuel Vimes has to deal with a murder, and some golems who are acting suspiciously, whilst the watch has gained a new member: a dwarf who may just be female.
Jingo (5 November, 1998)
Ankh-Morpork goes to war, but it doesn't have an army. The city watch are called in and together with the Patrician, set about saving the day. Nobby gets in touch with his feminine side.
The Fifth Elephant (4 November, 1999)
Samuel Vimes is on the run. Werewolves are all around and he is unfamiliar territory.
The Truth (2 November, 2000)
William de Worde prints Ankh-Morpork's first ever newspaper as the Patrician is the suspect in a very devious crime. The City Watch appear as bit-players: who needs policemen when you control the press?
Night Watch (1 October, 2003)
By far the darkest of the Vimes books: Vimes is whirled back in time. We may not know who polices the police, but do we find out who first led the leader.
Pyramids (June 1990)
The only place for the sons of gentlemen to get a decent education is at the Assassin's Guild, so where else should the son of the god-king (and his sister) go for his schooling? Unfortunately the Old Kingdom is not designed for people who can think. Pteppic and Ptraci ptake on mummies and philosophers.
Moving Pictures (14 November, 1991)
Ankh-Morpork goes mad as the magic of the silver screen comes to Holy Wood and suddenly everyone wants to be a star. Introduces Gaspode the Wonder Dog, who doesn't bark: he says "Woof".
Small Gods (27 May, 1993)
The great god Om is not happy. His followers no longer believe, and it is hard to get believers if you are currently manifesting as a small and bad-tempered tortoise. This is the tale of one novice priest and the Quisition.
Monstrous Regiment (1 October, 2003)
Pretty Polly Oliver joins the army to find her long-lost brother, but sisterhood is more powerful than she thinks.
The Amazing Maurice and his Educated Rodents (November 2001)
What happens to rats who eat things they find in the Wizards' rubbish tip? And what happens to the cat who eats one of the rats? Exactly. The cat who pays the pied piper calls the tune.
The Wee Free Men (1 May, 2003)
Another book for the bairns. There has never been a witch on the chalk downs, but up until now there has never been a need for a witch on the chalk downs.
The Science of Discworld - with Ian Stewart and Jack Cohen (1999)
Or more accurately, the Science of Roundworld: A slantways look at science through the eyes and poking fingers of the wizards of the Unseen University.
The Science of the Discworld II - The Globe - with Ian Stewart and Jack Cohen (2 May, 2002)
Rincewind, the Librarian and the entire Faculty of the Unseen University take us on a practical experiment in space and time. With foot-notes. Well, foot-chapters, actually.