Wikipedia has been cited for the first time in a reported judgment of the High Court of Justice. The case, Kay v. the Commissioner of Police of the Metropolis  EWHC 1536 (Admin) (27 June 2006), concerned Critical Mass, a regular meeting of cyclists who ride through London along an unplanned route. Critical Mass' counsel introduced the English Wikipedia entry on Critical Mass, quoted in paragraph 4 of the judgment, to show that the group was leaderless and had no goals other than cycling together.
The claimant was Des Kay, a participant in the Critical Mass rides. He won a declaration that there was no need to apply for permission from the Metropolitan Police Service before holding its events. The court was unable to accept there was definitely no campaigning motivation to Critical Mass rides, but went on to find that it is "not necessarily" true that they have a campaigning motivation. Ultimately, the claimant won a favourable declaration by demonstrating that there had been over 140 rides since April 1994, and, as a result, the meetings were held to be "commonly or customarily held", a specific exception to the requirement to give prior notice of a public procession under section 11 of the Public Order Act 1986.
While this is the first known citation of Wikipedia by the English High Court, Wikipedia has been mentioned in two other reported rulings in the Courts of England and Wales. The first was in BBT Thermotechnology UK Ltd v Brainfire Group  DRS 3931 (12 January 2006), a disputed internet registration claim decided by an independent expert under the Nominet UK Dispute Resolution Service. In pre-trial emails quoted in paragraph 6.4 of the decision, the original registrant of buderus.co.uk referred the other to the existence of Danny Buderus as a demonstration that the name Buderus "was not a fanciful term" but originated as a surname, and therefore the domain should not be transferred to the Boulter Buderus domestic boiler company, which had been trading since 1731. The ruling ordered that the domain name should be transferred.
Wikipedia was also cited by the Competition Appeal Tribunal in the case of Media Marketing & Promotions v. Office of Communications  CAT 12 (15 May 2006). Media Marketing & Promotions referred to the entry on rent (see paragraph 117 of the judgment) in an unsuccessful attempt to avoid action by OFCOM for contravention of the Communications Act 2003.
In the US, Wikipedia has been cited and discussed by courts on a number of occasions, most recently by the California Court of Appeal in the Apple v. Does case. For a list, see Wikipedia:Wikipedia as a court source.