Famous advertising agencies

BBDO — works with Anheuser-Busch, Visa, and PepsiCo.
Crispin Porter + Bogusky –famous for Subservient Chicken, works with Burger King, EarthLink, Virgin Atlantic Airways, Volkswagen
Doyle Dane Bernbach — created famous campaigns for Volkswagen (including the famous “Lemon” ad) and Avis Rental Cars (“We’re number 2. We try harder.”)
Goodby, Silverstein & Partners — famous for the “Got Milk?” campaign, among others
JWT (formerly J. Walter. Thompson) — works with Kelloggs, Unilever, Diageo.
The Martin Agency — UPS, GEICO, NASCAR, Miller (Lite, MGD), Hanes, and others
N.W. Ayer & Son — the first ad agency in the United States, coined “When it rains it pours” (Morton Salt), “A diamond is forever” (De Beers), “Reach out and touch someone” (AT&T), “Be all you can be” (United States Army), and others
Ogilvy & Mather — famous for the Rolls-Royce print ad with the headline “At 60 miles an hour the loudest noise in this new Rolls-Royce comes from the electric clock”, among other ads
Saatchi and Saatchi — most famous for working with the Conservative Party especially during the 1979 general election (Maurice and Charles Saatchi later left and set-up M&C Saatchi)
TBWA\Chiat\Day — works with Apple Computer (including the “Think Different” campaign) and adidas. Responsible for creating the fcuk brand and (in the UK) Wonderbra advertising.
Partnership Advertising — responsible for developing the “loading cancer” ad that won the 2003 New York Festivals Finalists

Largest Advertising Groups

According to the Research Company Evaluating the Media Agency Industry, the 2004 top 6 largest advertising groups ranked by worldwide billings were the following:

WPP Group: $48.055 Billion
Publicis: $34.365 Billion
Interpublic: $27.870 Billion
Omnicom Group: $25.230 Billion
Aegis Group: $20.355 Billion
Havas: $8.775 Billion