Joe Williams (1885)
Mickey Cochrane (1903)
Ernie Lombardi (1908)
Phil Regan (1937)
Marty Pattin (1943)
Bert Blyleven (1951)
Kenny Williams (1964)
Bret Boone (1969)
Lou Merloni (1971)
A star in the Negro Leagues, some say that Smokey Joe Williams was a better pitcher than Satchel Paige.
I haven't checked, but my guess is that four Hall-of-Famers born on the same day is the record.