Lead Every Day was founded by Mark Miller and Randy Gravitt. They have produced a plethora of leadership content, including the SERVE model, specifically for Chick-fil-A.
The Table Group was co-founded by Patrick Lencioni, who has written several excellent books on leadership, such as The Ideal Team Player and The Five Dysfunctions of a Team.
The Optimism Company is headed by Simon Sinek, a popular author and voice in the conversation about effective leadership. His works include Leaders Eat Last and Start with Why.
HBR has been around since 1922 and produces tons of content and resources for leadership in the workplace.