Everyone knows about the option to kill Michael or Trevor, but I'm interested to see if anyone has found any other options and different outcomes. Interesting ones I have found:
-In the Dom mission on Maze Bank, it is possible that Dom will not land on the truck. This triggers a cutscene where Dom drives to where the truck parked and yells at you and tells you to meet him at the Land Act Dam.
-In the last mission for Mark, when you arrive to the scene of the burnt down house, you don't have to kill Mark or the police.
Some of the more obvious ones:
-You can beat Lamar on the first mission (from what I have heard)
-Trevor doesn't have to kill the celebrity on the train tracks and can let him live
-You can let Dr. Freidlander escape. Either way, he is reported dead for some reason
-Some of the random events grant you money rewards or stock tips
-You can let Mrs. Mathers live after collecting all the underwater parts.