Financial services, especially stock and fund brokering.

Basically, the web blows up any business model in which "brokers" or "agents" or "middlemen" or "resellers" mark up goods or services, but add no value in the process.

Travel agents were incredibly vulnerable, as so many not only added no value to transactions but thrived by acting as firewall shields between customers and travel information. Stockbrokers and other "sell-by-system" people got hurt too. Even booksellers who don't know their own stock and can't add value by recommending titles.