When Christopher Reeve (Whom Robin Williams went to school with) had his tragic accident, he was in a constant state of depression. One day, a man came in wearing scrubs and a blue hat, claiming to be a proctologist who was there to give him a rectal exam. It was Robin Williams, fresh off the set of his new movie. For the first time since the accident, Christopher Reeve laughed. Robin Williams went all that way for an old classmate, even presenting Reeve with an actual blank check to help with his friend's medical expenses.
And then after both Reeve and his wife passed away, Williams took it upon himself to support their son.
When Williams did a stint for the USO, a bugle suddenly sounds and the entire crowd of military personnel turns away and salutes in its direction (known as a "Retreat" ceremony, notifying the end of the work day). After an initial reaction of surprise ("Uh-oh!"), Robin is quick to remove his hat and stand in silence until the ceremony is complete, after which the soldiers turn around and applaud. Robin is, understandably, relieved to find out that they did not turn around because of the quality of his stand-up.
...I'm not gonna forget that. I've never had an entire audience just go, "FORGET YOU!" [Turns away]
The commercial that he and his daughter did for the Ocarina of Time remake. Did we mention his daughter's name is Zelda?
When Robin Williams was young and breaking into show business, his father insisted he learn a trade, just in case this comedy thing didn't pan out. As a result, Williams became a certified welder. When he accepted his first Oscar, Williams mentioned his (now deceased) father's concerns in his acceptance speech. The way he talked about it was funny, but there was nothing insincere about Williams proudly holding his award aloft for his father to see.
Williams recorded all his voice work for Aladdin in the studio across the street from where they were filming Schindler's List, which is, of course, one of the most depressing movies of all time. At the end of every day, he would cross the street and cheer up the List cast and crew with his material.
The absolute flood of Twitter, Facebook, and Tumblr posts (as well as the flood of comments on countless other websites and forums) after his death, showing the sheer number of people who adored him and were inspired by him. To put it into perspective, it's more difficult to find a celebrity that hasn't said a kind word about him, proving just how many lives he's been a part of.
When alive, Robin would make sure to go and cheer up sick kids in hospital, specifically at St. Jude's. Due to this, his daughter Zelda asked mourning fans to donate money to said clinic in his memory.