If you want to roleplay with humans in a text-based adventure, there are many sites online that do that... a good site that I have used is RPOL (Roleplay Online). See the related link below. There are also software packages that do that if you already have a party. You could also try the Meetup site if you want to play online or meetup with humans. There are meetup groups that do both.
If you want a computer roleplaying game, Neverwinter Nights and Neverwinter Nights 2 use official D&D rules (close to them anyways 3 and 3.5 edition DnD rules). Both games offer an online client and many player-hosted worlds are available to play on, the community for both games is very active. There is also DDO which is the actual Dungeons and Dragons MMORPG.
There are also Dnd "style" fantasy games like 'World of Warcraft' or even 'Dungeons and Dragons Stormreach online', but they are very watered down action games basically, not really close to the complexity that DnD offers, but some people like fun and basic fantay-action games so maybe that's what you want.
There is a MUD text-based Discworld game (see related links).
Offline there are also several Discworld Board Games (such as "Thud!") and Discworld Video Games for PC, Playstation, etc (such as "Discworld Noir").
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.
Discworld 2 is a point-and-click adventure game released in 1996, which means it may not be available to play online legally. You may be able to find physical copies of the game for sale from various collectors or online marketplaces. Alternatively, you could try using emulation software to run the game on your computer.
The Discworld Companion has around 344 pages.
The Art of Discworld was created in 2004.
Discworld MUD happened in 1991.
Discworld MUD was created in 1991.
Discworld Noir happened in 1999.
GURPS Discworld happened in 1998.
GURPS Discworld was created in 1998.
The Science of Discworld was created in 1999.
It does. Try updating your Java version.
Sir Terry Pratchett wrote Discworld.