Internet

The Internet was a worldwide computer network on Earth. The Fourth Doctor called it "a 21st century human obsession". (AUDIO: Energy of the Daleks) The Internet was a useful research tool. (PROSE: The Dying Days, Instruments of Darkness, TV: Rose)

Internet cafés existed by 1997, (PROSE: The Dying Days)

UNIT maintained a website with restricted access sections that allowed remote launch of non-nuclear UK missiles. (TV: World War Three)

search-wise.net was a search engine operational in 2005. Rose Tyler used it to look for information about the Doctor, which led her to whoisdoctorwho.co.uk. (TV: Rose)

Mickey tracked information about the Doctor between 2005 and 2006. (TV: Aliens of London) The Ninth Doctor, realising this would make him noticed, devised a computer virus, designed to delete all references to the Doctor from the Internet, and asked Mickey to run the program. (TV: World War Three)

In 2006, Mr Cleaver put up his concerns about the safety of the Blaidd Drwg nuclear power plant on the Internet. Cathy Salt discovered these results. (TV: Boom Town)

In 2008, the Eleventh Doctor used a webchat feature to communicate with a number of world organisations using Jeff Angelo's laptop; he later advised Jeff to delete his Internet history. (TV: The Eleventh Hour) This incarnation of the Doctor had a particular disdain for the popular messaging service Twitter. (TV: The Power of Three)

In 2012, Henry van Statten was the self-proclaimed owner of the Internet. At that time, connections throughout the Internet were fast enough for a Dalek to download its entire contents within seconds. (TV: Dalek)

Also in 2012, the Fourth Doctor instructed Sarah Jane Smith and Harry Sullivan to browse the net, at a Prague internet café, for any information on a Mr Drak. He later scolded them for not thinking of translating the words "Bojan Novak" and "Ctislav" into English while they were there. (PROSE: The Dragons of Prague)

By 2025, GlobeSphere provided holographic Internet access, and charged for time spent online to one's personal account. Accounts were recognised by fingerprint. The system did not recognise the Fourth Doctor's fingerprint, until he used the sonic screwdriver on it. (AUDIO: Energy of the Daleks)

The Internet, in some form, was still in use on Earth in the very late 21st century, as webpages relating to the Bowie Base One crew and Adelaide Brooke's granddaughter existed. (TV: The Waters of Mars)

Donna Noble, desiring to go shopping, once asked the Tenth Doctor if the TARDIS had Broadband. (COMIC: The Greatest Mall in the Universe)

Behind the scenes
The Internet has played a major role in promoting both the classic and revival Doctor Who series since the late 1990s. The BBC's website offered original story content (Scream of the Shalka, Real Time, Death Comes to Time, etc.) for the Internet, with exclusive-to-Internet short stories also uploaded from time to time. The Doctor Who website Official Twitter accounts related to the franchise are also maintained. Both Torchwood and The Sarah Jane Adventures had official websites and exclusive online content uploaded over the years they were running, as well.

When the series returned to TV in 2005, the BBC launched a "viral marketing" campaign by creating faux websites based upon characters and organisations featured in the series, such as the aforementioned UNIT site, which fans could visit. The primary viral site during the 2005 season was Mickey's Defending the Earth! site, which was updated each week with teasers or background information (all presented in an in-universe style) relating to that week's episode. The site included a message board that occasionally featured messages from recognisable characters, such as Sarah Jane Smith a year before she returned to the series in School Reunion.

A large number of books and audio dramas are currently available for download via the Internet, with a growing number of official and unofficial books also being made available only as e-books and thus available only via the Internet.

The search-wise.net website used by Rose actually exists. It is a "dummy site" for use by movie and TV producers in lieu of using recognisable sites like Google, although, increasingly since the start of the Steven Moffat era of the programme, explicit references have been made in Doctor Who scripts to real-life websites such as Facebook, Twitter, Flickr. Tumblr is seen being used on an iPhone in Kill the Moon.