O. Offliberty
Not the most well-known site in the world, but a highly useful one, which allows users to make online content viewable and usable on their computers without the need for an internet connection, making it particularly useful for long journeys, amongst other things. Honourable mentions go to the Onion, a satirical news website that is frequently mistaken as serious by everybody from politicians to presenters, and Outlook, Microsofts email client that has recently reinvented itself as a worthy competitor to Gmail after years of lagging behind it with their Hotmail service.
P. Pirate Bay
For many years, governments and industry bodies have tried to shut down The Pirate Bay, a Swedish site that distributes torrent files of questionable legality, and every time they have failed. Though its nature cannot be condoned, it cannot be denied that it has left its mark on the contemporary web. An honourable mention goes to Paypal, a money transfer service utilised by sites like eBay in order to ensure that the financial details of users are kept safe and not misused.