This page is really old. I keep it alive because it has become a a period piece. (It was last updated January 2000.)
At the time it listed some of the larger web design firms that designed and developed corporate sites.
Included are a few of their clients, or sites that they worked on.
See their pages for more accurate information (where links still work).
San Francisco Bay Area Firms
Sapient E*TRADE, United, Hallmark, Nordstrom
Organic Inc. CDNow,DaimlerChrysler, Mastercard, Compaq
Verso Amazon, Cisco, Microsoft, Sony
WorldMind Media 3Com, Saturn,SGI, Sun Microsystems
vivid studios   Altavista, Schwab, Nike, Oraclek
Pear Transmedia Bugle Boy, Mattel, Broderbund
OnlineFocus FedEx, Apple, Sun, Sony
Darwin Digital Dupont, Lucent, Macromedia, Toyota
Key Point Software
Adaptec, Hewlett Packard
Other Firms
Razorfish (NY) AT&T, CBS, IBM, Time Warner, Sony, Pepsi, Disney
Plumb Design (NY) National Geographic, Smithsonian Institute
Rare Medium Group Inc (NY) HBO, Sharp, Sony, Lucent, Microsoft
Agency.com (NY) American Express, British Airways, Ford, Eastman-Kodak
Acro Media Inc. (Canada)