"One of the most important principles of Islam is that all things belong to God, and that wealth is therefore held by human beings in trust."
Zakat, meaning purification and growth, is one of the Five Pillars of Islam. Each Muslim calculates his net worth every year and donates around 2.5% to charity.
Some very charitable people donate as much as they want as sadaqa, or voluntary charity. This is usually done in secret.