Milano, Applegate, Macchio and Haim all had their breakthrough roles in the 1980s.
Haim, whose struggles to kick drugs overshadowed his once-bright acting career, died at the age of 38. An autopsy is to be conducted to determine the cause of his death, but police said they suspect the reason was an accidental drug overdose.
"Just woke up to the sad, sad news that Corey Haim passed away. RIP sweet boy," People.com reported Milano, Haim's ex-girlfriend, said via Twitter.
"Childhood hero," was actor-producer Ashton Kutcher's simple tweet of condolence.
"My thoughts go out to Corey's family and friends today. So sad," Applegate said on her Twitter page.
"Always so sad and disturbing when the addiction and demons prevail. RIP Corey Haim," Macchio Tweeted.
The A&E television network, which aired "The Two Coreys" reality show from 2007-08, posted a message on its Web site saying: "We are saddened by the tragic loss of Corey Haim who we had the pleasure to work with on the series 'The Two Coreys.' Our thoughts are with his family during this difficult time."
Haim starred in the show with his long-time friend and former frequent collaborator Corey Feldman.
Feldman's official Web site was not in service Wednesday after the news of Haim's death broke.