Yahoo, Twitter and LinkedIn have asked users to change their details, days after gossip site Gawker was hacked. Online game World of Warcraft, which has more than 12 million subscribers, has also asked some users to reset their passwords.
Although thousands of Twitter accounts were compromised after the attack, there have been few other reports of damage directly linked to the breach.
The attack on Gawker, which runs one of the world's most popular blog networks, was carried out over the weekend by an organization calling itself Gnosis. The group - which says it was making a protest at the site's perceived "arrogance" - subsequently published account details of 1.3 million Gawker users online, including a significant number of passwords.
Documents show that the most popular password among Gawker users was "123456", followed by "password" and "12345678". Although security experts warn against the use of passwords that are easy to guess, research suggests such behavior is increasingly common online.