The Knights ナイツ (Knights of the Eastern Calculus)
The Knights are an anonymous group of elite hackers in the Wired whose members do not know one another. They are behind the Psyche processor that turns Lain's NAVI into something far beyond a consumer machine, and they are linked to Accela and to the KIDS experiment. Their name plays on the 'Knights of the Lambda Calculus', an MIT hacker in-joke. Taro runs errands for them in exchange for gear. Ueda, asked at Otakon 2000 about a monitoring program called Knights, said the name only meant knights in the traditional sense, and that the real-world technical detail throughout the show was all the scenario writer's research. Konaka filled the script book with footnotes on the technology the Knights use.
More (spoilers)
Eiri uses the Knights as his believers, the faithful who make him a god. Lain finds and publishes their membership list on the Wired; the Men in Black kill some members and others die by suicide, reported worldwide as a wave of network-related deaths. With his believers gone, Eiri's hold weakens.
Sources: en.wikipedia.org archived · ja.wikipedia.org archived · animenewsnetwork.com archived · lawmune.net · konaka.com archived · lain.wiki · lain.wiki