How do I find someone's birthday for free on the internet?

Whatever makes you think you can? You can try googling someone's name. But such records are generally not freely available.

Actually there is a free birthday search, I have tried it on several people, and it has gotten all of them right

Free Birthday Database

Also on the Zappa search it will often list a birth day by the search name.

Whoops. Checked out the site and its is very scary. Too much info to make freely available to the public.