MOBA
It began as a map built inside someone else's game. Aeon of Strife (2002), a custom StarCraft map, set the pattern of controlling a single hero to attack down three lanes lined with towers, and Eul carried that into Warcraft III as Defense of the Ancients. It passed from one modder's hands to the next -- Guinsoo refined it, and the anonymous IceFrog developed it for years. From there came League of Legends in 2009 and Dota 2 in 2013. The largest esport in the world grew up out of something that was nobody's business plan. It is the biggest single example of the lineage where mods created a profession (see MODs and Steam).
The myth, corrected
League of Legends was made by the people who created DotA striking out on their own
Brandon Beck and Marc Merrill, who founded Riot Games, were classmates and roommates at the University of Southern California -- not DotA's developers, but players who were obsessed with it. They started a company because they wanted to make a real business out of it, and only afterward hired the developer of DotA Allstars, Guinsoo, and its community manager, Pendragon. The people who made it did not go independent; the people who played it started a company and brought the makers in. The order was reversed.