The donation will come from the $2bn William H Gates Foundation, which was established three years ago to support world health, population and education issues.
Mr Gates, the richest man in America, used to be criticised for not embracing charitable causes, but has more recently appeared to model himself on the legendary philanthropist Andrew Carnegie, who established the system of free public libraries in the US.
As well as the Gates Foundation, the Gateses have established the Gates Library Foundation to help libraries in poor districts to obtain computer equipment and access to the Internet.
While the gesture has been widely appreciated, the timing of their latest gift suggests that altruism may not have been the only motive.
Bill Gates is under pressure in the lawsuit brought against Microsoft by the US Justice Department, with almost daily criticism in the US media of his hesitant and evasive testimony - shown to the court in videotaped instalments.
But the enormous changes that would be wrought in the industry by America Online's acquisition of Netscape have given Mr Gates ammunition to seek to have the case dropped. AOL is the dominant provider of Internet services in America, and Netscape is the other supplier of web browsers.
The company's top lawyers yesterday sought to have the competition case dropped.Reuse content