The phrase pro bono is useful for describing things you do to help people, rather than to earn money. For example, a dentist might offer free teeth cleaning as pro bono work for low-income patients.
You're most likely to see the term pro bono used in reference to free legal services. Many attorneys offer pro bono legal aid either to help poor clients or because the outcome of a case would serve the public good. Pro bono is short for pro bono publico, literally "for the public good" in Latin.