Whenever you are in the process of connecting to server, there is always a small list of top server supporters, along with a number. What exactly makes someone a "supporter" of a particular server, and what actions move you up in the list?
The list that you see while loading a certain map is the list of people that donated to that map's author, by buying the correspondent map stamp in the Mann Co. in-game store.
Map stamps were introduced during Australian Christmas Update. For more info you can check the official TF2 Wiki.
Note that this happens only when loading community created maps. When loading official Valve maps loading screen usually displayed the ladder of mini-duel winners.