Check out the TF2 Workshop Stormy. There are millions of entries for new weapons, items, paints, effects, miscs and otherstuff, all community-made. People model them, create them, and submit for approval. Then if an item is really popular, Valve will actually add it into a future update for TF2. The new item will be in game, and any sale of that item through the mann. co. store will benefit the creator, since they get a cut of the revenue.