There is but it's a MUD (text based game).
There are also several Discworld Board Games (such as "Thud!") and Discworld Video Games for PC, Playstation, etc (such as "Discworld Noir").
There have been a few different computer game adaptations. The best are argueably Discworld & Discworld II, point and click adventure games released on the PC and playstation the mid 90s. Both were well received by fans for representing the discworld well, and both included a great voice cast with Eric Idle playing rincewind and tony Robinson and Jon pertwee featuring in the first.
There is a text-based MUD Discworld game online. Other than that, you will need to purchase the official games on a disc to play.
The Discworld Companion has 288 pages.
The Art of Discworld was created in 2004.
Discworld MUD happened in 1991.
Discworld MUD was created in 1991.
GURPS Discworld happened in 1998.
GURPS Discworld was created in 1998.
Discworld Noir happened in 1999.
The Science of Discworld was created in 1999.
Sir Terry Pratchett wrote Discworld.
The Discworld Mapp was created on 1995-11-09.