The star donated royalties from some of his biggest selling singles and is said to have given a gameshow contestant thousands of pounds to fund her IVF treatment.
He has helped countless children as a result of his donations to Childline and also supported other organisations including the Terrence Higgins Trust and Macmillan Cancer Support.
Michael, who spoke about losing his partner Anselmo Feleppa to HIV, “personally supported” the Terrence Higgins Trust for “many years”, Jane Barron from the organisation said.
“We are so saddened by the loss of George Michael,” she added.
“George also often thought of us to kindly donate experiences and gifts that were used to raise vital funds to help us support people living with HIV.
“Along with other charities, we were grateful to benefit from the royalties of George’s 1991 duet with Elton John, Don’t Let the Sun Go Down On Me.
“His donations contributed to a vision of a world where people living with HIV live healthy lives free from prejudice and discrimination.
“Thanks to George’s legacy, we are a step closer to that world and we are so grateful for his support and friendship over the years.
“Our thoughts are with his family and loved ones at this difficult time.”
Childline founder and president Dame Esther Rantzen revealed that Michael had given the royalties from his 1996 number one single Jesus To A Child to the charity.
She said: “For years now he has been the most extraordinarily generous philanthropist, giving money to Childline, but he was determined not to make his generosity public so no-one outside the charity knew how much he gave to the nation’s most vulnerable children.
“Over the years he gave us millions and we were planning next year, as part of our 30th anniversary celebrations to create, we hoped, a big concert in tribute to him - to his artistry, to his wonderful musicality but also to thank him for the 100s of 1,000s of children he helped through supporting Childline.
“And it is particularly tragic that Christmas, which was when he released Jesus to a Child, would also be the time when we lost him.
“I think all of us have memories of particular Wham! songs and George Michael songs which mean a great deal to us.
“Certainly, for Britain’s children, George Michael meant so much more.”
In 1997 Michael lost his mother Lesley to cancer - at the time he described her as a “woman of great compassion”, adding: “She felt much as I do, that we were living in a world that was gradually being drained of that.”
Almost 10 years after her death he played a special free concert in north London for NHS nurses as thanks for the care they gave her and over the years continued to support charities such as Macmillan Cancer Support.
Lynda Thomas, chief executive of Macmillan Cancer Support, said: “We are deeply saddened to hear about the death of George Michael who as a Macmillan Ambassador provided his committed support to us in a number of ways.
“We are extremely grateful to George and send our condolences to his family, friends and fans.”
And in the hours after the singer’s death was announced TV presenter Richard Osman revealed that Michael had called a woman who appeared on Deal or No Deal to give her the money she needed for IVF treatment.
Osman tweeted: “A woman on ‘Deal Or No Deal’ told us she needed £15k for IVF treatment. George Michael secretly phoned the next day and gave her the £15k.”
Michael was also among the stars involved in the original Band Aid single Do They Know It’s Christmas? which raised more than £19 million for famine relief in Ethiopia after selling more than two million copies worldwide.